Source : Webster's Revised Unabridged Dictionary (1913)

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

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

Source : WordNet®

thickened
     adj 1: made or having become thick; "thickened bronchial arteries"
     2: having skin made tough and thick through wear [syn: {calloused}]
     3: made thick in consistency; "flour-thickened gravy";
        "dust-thickened saliva"