The Collaborative International Dictionary of English v.0.48:

Thicken \Thick"en\, v. i.
   To become thick. "Thy luster thickens when he shines by."
   --Shak.
   [1913 Webster]

         The press of people thickens to the court. --Dryden.
   [1913 Webster]

         The combat thickens, like the storm that flies.
                                                  --Dryden.
   [1913 Webster]

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):

thicken
    v 1: make thick or thicker; "Thicken the sauce"; "inspissate the
         tar so that it becomes pitch" [syn: thicken,
         inspissate] [ant: thin]
    2: become thick or thicker; "The sauce thickened"; "The egg yolk
       will inspissate" [syn: thicken, inspissate] [ant: thin]
    3: make viscous or dense; "thicken the sauce by adding flour"
       [syn: thicken, inspissate]

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "thicken":
   add to, aggrandize, amplify, augment, beat up, boost, broaden,
   build, build up, cake, candy, churn, clabber, clot, clump, cluster,
   coagulate, colloid, colloidize, concrete, condense, congeal, cream,
   crystallize, curd, curdle, emulsify, emulsionize, enlarge, exalt,
   expand, extend, fatten, fill out, firm up, gel, gelatinate,
   gelatinize, granulate, grow thick, harden, hike, hike up,
   incrassate, increase, inflate, inspissate, jack up, jell, jellify,
   jelly, jump up, knot, lengthen, lopper, lump, maximize, parlay,
   put up, pyramid, raise, set, solidify, stiffen, take a set, thick,
   up, whip