The Collaborative International Dictionary of English v.0.48:

Thicken \Thick"en\, v. t. [imp. & p. p. Thickened; p. pr. &
   vb. n. Thickening.]
   To make thick (in any sense of the word). Specifically:
   [1913 Webster]
   (a) To render dense; to inspissate; as, to thicken paint.
       [1913 Webster]
   (b) To make close; to fill up interstices in; as, to thicken
       cloth; to thicken ranks of trees or men.
       [1913 Webster]
   (c) To strengthen; to confirm. [Obs.]
       [1913 Webster]

             And this may to thicken other proofs. --Shak.
       [1913 Webster]
   (d) To make more frequent; as, to thicken blows.
       [1913 Webster]

WordNet (r) 3.0 (2006):

thickened
    adj 1: made or having become thick; "thickened bronchial
           arteries"
    2: having calluses; having skin made tough and thick through
       wear; "calloused skin"; "with a workman's callous hands"
       [syn: calloused, callous, thickened]
    3: made thick in consistency; "flour-thickened gravy"; "dust-
       thickened saliva"

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "thickened":
   adhesive, amylaceous, caked, cakey, clabbered, clammy, clotted,
   coagulated, congealed, curded, curdled, doughy, gaumy, gelatinized,
   gelatinous, glairy, gluelike, gluey, glutenous, glutinose,
   glutinous, gooey, grumous, gumbo, gumbolike, gumlike, gummous,
   gummy, heavy, incrassate, inspissate, inspissated, jelled, jellied,
   jellylike, lumpish, lumpy, mucilaginous, pasty, ropy, slabby,
   slimy, slithery, starchy, sticky, stodgy, stringy, syrupy, tacky,
   tenacious, thick, tough, tremelloid, tremellose, viscid, viscose,
   viscous