The Collaborative International Dictionary of English v.0.48:

Tan \Tan\, v. t. [imp. & p. p. Tanned; p. pr. & vb. n.
   Tanning.] [F. tanner, LL. tannare. See Tan, n.]
   1. To convert (the skin of an animal) into leather, as by
      usual process of steeping it in an infusion of oak or some
      other bark, whereby it is impregnated with tannin, or
      tannic acid (which exists in several species of bark), and
      is thus rendered firm, durable, and in some degree
      impervious to water.
      [1913 Webster]

   Note: The essential result in tanning is due to the fact that
         the tannins form, with gelatins and albuminoids, a
         series of insoluble compounds which constitute leather.
         Similar results may be produced by the use of other
         reagents in place of tannin, as alum, and some acids or
         chlorides, which are employed in certain processes of
         tanning.
         [1913 Webster]

   2. To make brown; to imbrown, as by exposure to the rays of
      the sun; as, to tan the skin.
      [1913 Webster]

   3. To thrash or beat; to flog; to switch; as, to tan a
      disobedient child's hide. [Colloq.]
      [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

tanned
    adj 1: (of skin) having a tan color from exposure to the sun; "a
           young bronzed Apollo" [syn: bronzed, suntanned,
           tanned]
    2: converted to leather by a tanning agent [ant: untanned]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "tanned":
   beige, brown, brownish, brownish-yellow, brunet, chocolate,
   cinnamon, cocoa, cocoa-brown, coffee, coffee-brown, drab, dun,
   dun-brown, dun-drab, ecru, fawn, fawn-colored, fuscous, grege,
   hazel, khaki, lurid, nut-brown, olive-brown, olive-drab, seal,
   seal-brown, sepia, snuff-colored, sorrel, tan, taupe, tawny, toast,
   toast-brown, umber, umber-colored, walnut, walnut-brown,
   yellowish-brown