The Collaborative International Dictionary of English v.0.48:

Coxcomb \Cox"comb`\ (k?ks"k?m`), n. [A corrupted spelling of
   cock's comb.]
   1.
      (a) A strip of red cloth notched like the comb of a cock,
          which licensed jesters formerly wore in their caps.
      (b) The cap itself.
          [1913 Webster]

   2. The top of the head, or the head itself.
      [1913 Webster]

            We will belabor you a little better,
            And beat a little more care into your coxcombs.
                                                  --Beau & Fl.
      [1913 Webster]

   3. A vain, showy fellow; a conceited, silly man, fond of
      display; a superficial pretender to knowledge or
      accomplishments; a fop.
      [1913 Webster]

            Fond to be seen, she kept a bevy
            Of powdered coxcombs at her levee.    --Goldsmith.
      [1913 Webster]

            Some are bewildered in the maze of schools,
            And some made coxcombs, nature meant but fools.
                                                  --Pope.
      [1913 Webster]

   4. (Bot.) A name given to several plants of different genera,
      but particularly to Celosia cristata, or garden
      cockscomb. Same as Cockscomb.
      [1913 Webster]

WordNet (r) 3.0 (2006):

coxcomb
    n 1: a conceited dandy who is overly impressed by his own
         accomplishments [syn: coxcomb, cockscomb]
    2: a cap worn by court jesters; adorned with a strip of red
       [syn: cockscomb, coxcomb]
    3: the fleshy red crest on the head of the domestic fowl and
       other gallinaceous birds [syn: comb, cockscomb,
       coxcomb]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "coxcomb":
   Beau Brummel, beau, bladder, blade, blood, boulevardier, buck,
   cap and bells, clotheshorse, dandy, dude, exquisite, fashion plate,
   fine gentleman, fop, fribble, gallant, jack-a-dandy, jackanapes,
   lady-killer, lounge lizard, macaroni, man-about-town, masher,
   motley, puppy, slapstick, sock, spark, sport, swell