The Collaborative International Dictionary of English v.0.48:

Under \Un"der\ ([u^]n"d[~e]r), prep. [AS. under, prep. & adv.;
   akin to OFries. under, OS. undar, D. onder, G. unter, OHG.
   untar, Icel. undir, Sw. & Dan. under, Goth. undar, L. infra
   below, inferior lower, Skr. adhas below. [root]201. Cf.
   Inferior.]
   1. Below or lower, in place or position, with the idea of
      being covered; lower than; beneath; -- opposed to over;
      as, he stood under a tree; the carriage is under cover; a
      cellar extends under the whole house.
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            Fruit put in bottles, and the bottles let down into
            wells under water, will keep long.    --Bacon.
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            Be gathered now, ye waters under heaven,
            Into one place.                       --Milton.
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   2. Hence, in many figurative uses which may be classified as
      follows;
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      (a) Denoting relation to some thing or person that is
          superior, weighs upon, oppresses, bows down, governs,
          directs, influences powerfully, or the like, in a
          relation of subjection, subordination, obligation,
          liability, or the like; as, to travel under a heavy
          load; to live under extreme oppression; to have
          fortitude under the evils of life; to have patience
          under pain, or under misfortunes; to behave like a
          Christian under reproaches and injuries; under the
          pains and penalties of the law; the condition under
          which one enters upon an office; under the necessity
          of obeying the laws; under vows of chastity.
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                Both Jews and Gentiles . . . are all under sin.
                                                  --Rom. iii. 9.
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                That led the embattled seraphim to war
                Under thy conduct.                --Milton.
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                Who have their provand
                Only for bearing burdens, and sore blows
                For sinking under them.           --Shak.
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      (b) Denoting relation to something that exceeds in rank or
          degree, in number, size, weight, age, or the like; in
          a relation of the less to the greater, of inferiority,
          or of falling short.
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                Three sons he dying left under age. --Spenser.
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                Medicines take effect sometimes under, and
                sometimes above, the natural proportion of their
                virtue.                           --Hooker.
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                There are several hundred parishes in England
                under twenty pounds a year.       --Swift.
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                It was too great an honor for any man under a
                duke.                             --Addison.
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   Note: Hence, it sometimes means at, with, or for, less than;
         as, he would not sell the horse under sixty dollars.
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               Several young men could never leave the pulpit
               under half a dozen conceits.       --Swift.
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      (c) Denoting relation to something that comprehends or
          includes, that represents or designates, that
          furnishes a cover, pretext, pretense, or the like; as,
          he betrayed him under the guise of friendship;
          Morpheus is represented under the figure of a boy
          asleep.
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                A crew who, under names of old renown . . .
                abused
                Fanatic Egypt.                    --Milton.
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                Mr. Duke may be mentioned under the double
                capacity of a poet and a divine.  --Felton.
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                Under this head may come in the several contests
                and wars betwixt popes and the secular princes.
                                                  --C. Leslie.
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      (d) Less specifically, denoting the relation of being
          subject, of undergoing regard, treatment, or the like;
          as, a bill under discussion.
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                Abject and lost, lay these, covering the flood,
                Under amazement of their hideous change.
                                                  --Milton.
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   Under arms. (Mil.)
      (a) Drawn up fully armed and equipped.
      (b) Enrolled for military service; as, the state has a
          million men under arms.

   Under canvas.
      (a) (Naut.) Moved or propelled by sails; -- said of any
          vessel with her sail set, but especially of a steamer
          using her sails only, as distinguished from one under
          steam. Under steam and canvas signifies that a vessel
          is using both means of propulsion.
      (b) (Mil.) Provided with, or sheltered in, tents.

   Under fire, exposed to an enemy's fire; taking part in a
      battle or general engagement.

   Under foot. See under Foot, n.

   Under ground, below the surface of the ground.

   Under one's signature, with one's signature or name
      subscribed; attested or confirmed by one's signature. Cf.
      the second Note under Over, prep.

   Under sail. (Naut.)
      (a) With anchor up, and under the influence of sails;
          moved by sails; in motion.
      (b) With sails set, though the anchor is down.
      (c) Same as Under canvas
      (a), above. --Totten.

   Under sentence, having had one's sentence pronounced.

   Under the breath, Under one's breath, with low voice;
      very softly.

   Under the lee (Naut.), to the leeward; as, under the lee of
      the land.

   Under the gun. Under psychological pressure, such as the
      need to meet a pressing deadline; feeling pressured

   Under water, below the surface of the water.

   Under way, or Under weigh (Naut.), in a condition to make
      progress; having started.
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The Collaborative International Dictionary of English v.0.48:

Under \Un"der\, a.
   Lower in position, intensity, rank, or degree; subject;
   subordinate; -- generally in composition with a noun, and
   written with or without the hyphen; as, an undercurrent;
   undertone; underdose; under-garment; underofficer;
   undersheriff.
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   Under covert (Zool.), one of the feathers situated beneath
      the bases of the quills in the wings and tail of a bird.
      See Illust. under Bird.
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The Collaborative International Dictionary of English v.0.48:

Under \Un"der\ ([u^]n"d[~e]r), adv.
   In a lower, subject, or subordinate condition; in subjection;
   -- used chiefly in a few idiomatic phrases; as, to bring
   under, to reduce to subjection; to subdue; to keep under, to
   keep in subjection; to control; to go under, to be
   unsuccessful; to fail; to go bankrupt.
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         I keep under my body, and bring it into subjection. --1
                                                  Cor. ix. 27.
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         The minstrel fell, but the foeman's chain
         Could not bring his proud soul under.    --Moore.
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   Note: Under is often used in composition with a verb to
         indicate lowness or inferiority in position or degree,
         in the act named by the verb; as, to underline; to
         undermine; to underprop.
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WordNet (r) 3.0 (2006):

under
    adv 1: down to defeat, death, or ruin; "their competitors went
           under"
    2: through a range downward; "children six and under will be
       admitted free"
    3: into unconsciousness; "this will put the patient under"
    4: in or into a state of subordination or subjugation; "we must
       keep our disappointment under"
    5: below some quantity or limit; "fifty dollars or under"
    6: below the horizon; "the sun went under"
    7: down below; "get under quickly!"
    8: further down; "see under for further discussion" [syn:
       under, below]
    adj 1: located below or beneath something else; "nether
           garments"; "the under parts of a machine" [syn: nether,
           under]
    2: lower in rank, power, or authority; "an under secretary"

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "under":
   answerable to, at a disadvantage, at the nadir, below, below deck,
   below par, below the mark, belowstairs, beneath, collateral,
   dependent, down, down below, downstairs, drunk, earlier, high,
   impaired, in the gutter, inferior, infra, least, least of all,
   less, lesser, low, lower, lowest, neath, nether, out of sight,
   secondary, short of, sub, subjacent, subject, subordinate to,
   tipsy, tributary, under par, under the influence, underfoot,
   underneath, underwater