The Collaborative International Dictionary of English v.0.48:

Thin \Thin\, a. [Compar. Thiner; superl. Thinest.] [OE.
   thinne, thenne, thunne, AS. [thorn]ynne; akin to D. dun, G.
   d["u]nn, OHG. dunni, Icel. [thorn]unnr, Sw. tunn, Dan. tynd,
   Gael. & Ir. tana, W. teneu, L. tenuis, Gr. ? (in comp.)
   stretched out, ? stretched, stretched out, long, Skr. tanu
   thin, slender; also to AS. ?enian to extend, G. dehnen, Icel.
   ?enja, Goth. ?anjan (in comp.), L. tendere to stretch, tenere
   to hold, Gr. ? to stretch, Skr. tan. [root]51 & 237. Cf.
   Attenuate, Dance, Tempt, Tenable, Tend to move,
   Tenous, Thunder, Tone.]
   1. Having little thickness or extent from one surface to its
      opposite; as, a thin plate of metal; thin paper; a thin
      board; a thin covering.
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   2. Rare; not dense or thick; -- applied to fluids or soft
      mixtures; as, thin blood; thin broth; thin air. --Shak.
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            In the day, when the air is more thin. --Bacon.
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            Satan, bowing low
            His gray dissimulation, disappeared,
            Into thin air diffused.               --Milton.
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   3. Not close; not crowded; not filling the space; not having
      the individuals of which the thing is composed in a close
      or compact state; hence, not abundant; as, the trees of a
      forest are thin; the corn or grass is thin.
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            Ferrara is very large, but extremely thin of people.
                                                  --Addison.
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   4. Not full or well grown; wanting in plumpness.
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            Seven thin ears . . . blasted with the east wind.
                                                  --Gen. xli. 6.
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   5. Not stout; slim; slender; lean; gaunt; as, a person
      becomes thin by disease.
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   6. Wanting in body or volume; small; feeble; not full.
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            Thin, hollow sounds, and lamentable screams.
                                                  --Dryden.
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   7. Slight; small; slender; flimsy; wanting substance or depth
      or force; superficial; inadequate; not sufficient for a
      covering; as, a thin disguise.
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            My tale is done, for my wit is but thin. --Chaucer.
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   Note: Thin is used in the formation of compounds which are
         mostly self-explaining; as, thin-faced, thin-lipped,
         thin-peopled, thin-shelled, and the like.
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   Thin section. See under Section.
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The Collaborative International Dictionary of English v.0.48:

Thin \Thin\, adv.
   Not thickly or closely; in a seattered state; as, seed sown
   thin.
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         Spain is thin sown of people.            --Bacon.
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The Collaborative International Dictionary of English v.0.48:

Thin \Thin\, v. t. [imp. & p. p. Thinned; p. pr. & vb. n.
   Thinning.] [Cf. AS. ge[thorn]ynnian.]
   To make thin (in any of the senses of the adjective).
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The Collaborative International Dictionary of English v.0.48:

Thin \Thin\, v. i.
   To grow or become thin; -- used with some adverbs, as out,
   away, etc.; as, geological strata thin out, i. e., gradually
   diminish in thickness until they disappear.
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WordNet (r) 3.0 (2006):

thin
    adv 1: without viscosity; "the blood was flowing thin" [syn:
           thinly, thin] [ant: thick, thickly]
    adj 1: of relatively small extent from one surface to the
           opposite or in cross section; "thin wire"; "a thin
           chiffon blouse"; "a thin book"; "a thin layer of paint"
           [ant: thick]
    2: lacking excess flesh; "you can't be too rich or too thin";
       "Yon Cassius has a lean and hungry look"-Shakespeare [syn:
       thin, lean] [ant: fat]
    3: very narrow; "a thin line across the page" [syn: slender,
       thin]
    4: not dense; "a thin beard"; "trees were sparse" [syn:
       sparse, thin]
    5: relatively thin in consistency or low in density; not
       viscous; "air is thin at high altitudes"; "a thin soup";
       "skimmed milk is much thinner than whole milk"; "thin oil"
       [ant: thick]
    6: (of sound) lacking resonance or volume; "a thin feeble cry"
       [ant: full]
    7: lacking spirit or sincere effort; "a thin smile"
    8: lacking substance or significance; "slight evidence"; "a
       tenuous argument"; "a thin plot"; a fragile claim to fame"
       [syn: flimsy, fragile, slight, tenuous, thin]
    v 1: lose thickness; become thin or thinner [ant: inspissate,
         thicken]
    2: make thin or thinner; "Thin the solution" [ant: inspissate,
       thicken]
    3: lessen the strength or flavor of a solution or mixture; "cut
       bourbon" [syn: dilute, thin, thin out, reduce, cut]
    4: take off weight [syn: reduce, melt off, lose weight,
       slim, slenderize, thin, slim down] [ant: gain, put
       on]