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.
      [1913 Webster]

            Fruit put in bottles, and the bottles let down into
            wells under water, will keep long.    --Bacon.
      [1913 Webster]

            Be gathered now, ye waters under heaven,
            Into one place.                       --Milton.
      [1913 Webster]

   2. Hence, in many figurative uses which may be classified as
      follows;
      [1913 Webster]
      (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.
          [1913 Webster]
          [1913 Webster]

                Both Jews and Gentiles . . . are all under sin.
                                                  --Rom. iii. 9.
          [1913 Webster]

                That led the embattled seraphim to war
                Under thy conduct.                --Milton.
          [1913 Webster]

                Who have their provand
                Only for bearing burdens, and sore blows
                For sinking under them.           --Shak.
          [1913 Webster]
      (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.
          [1913 Webster]

                Three sons he dying left under age. --Spenser.
          [1913 Webster]

                Medicines take effect sometimes under, and
                sometimes above, the natural proportion of their
                virtue.                           --Hooker.
          [1913 Webster]

                There are several hundred parishes in England
                under twenty pounds a year.       --Swift.
          [1913 Webster]

                It was too great an honor for any man under a
                duke.                             --Addison.
          [1913 Webster]

   Note: Hence, it sometimes means at, with, or for, less than;
         as, he would not sell the horse under sixty dollars.
         [1913 Webster]

               Several young men could never leave the pulpit
               under half a dozen conceits.       --Swift.
         [1913 Webster]
      (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.
          [1913 Webster]

                A crew who, under names of old renown . . .
                abused
                Fanatic Egypt.                    --Milton.
          [1913 Webster]

                Mr. Duke may be mentioned under the double
                capacity of a poet and a divine.  --Felton.
          [1913 Webster]

                Under this head may come in the several contests
                and wars betwixt popes and the secular princes.
                                                  --C. Leslie.
          [1913 Webster]
      (d) Less specifically, denoting the relation of being
          subject, of undergoing regard, treatment, or the like;
          as, a bill under discussion.
          [1913 Webster]

                Abject and lost, lay these, covering the flood,
                Under amazement of their hideous change.
                                                  --Milton.
          [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

under fire
    adj 1: subjected to enemy attack or censure; "an official under
           fire for mismanagement" [syn: under attack(p), under
           fire(p)]