The Collaborative International Dictionary of English v.0.48:

Clown \Clown\, v. i.
   To act as a clown; -- with it. [Obs.]
   [1913 Webster]

         Beshrew me, he clowns it properly indeed. --B. Jonson.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Clown \Clown\ (kloun), n. [Cf. Icel. klunni a clumsy, boorish
   fellow, North Fries. kl["o]nne clown, dial. Sw. klunn log,
   Dan. klunt log, block, and E. clump, n.]
   1. A man of coarse nature and manners; an awkward fellow; an
      ill-bred person; a boor. --Sir P. Sidney.
      [1913 Webster]

   2. One who works upon the soil; a rustic; a churl.
      [1913 Webster]

            The clown, the child of nature, without guile.
                                                  --Cowper.
      [1913 Webster]

   3. The fool or buffoon in a play, circus, etc.
      [1913 Webster]

            The clown shall make those laugh whose lungs are
            tickle o'the sere.                    --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

clown
    n 1: a rude or vulgar fool [syn: clown, buffoon]
    2: a person who amuses others by ridiculous behavior [syn:
       clown, buffoon, goof, goofball, merry andrew]
    v 1: act as or like a clown [syn: clown, clown around,
         antic]

Moby Thesaurus II by Grady Ward, 1.0:

166 Moby Thesaurus words for "clown":
   Babbitt, Columbine, Hanswurst, Harlequin, Pantalone, Pantaloon,
   Philistine, Polichinelle, Pulcinella, Punch, Punchinello,
   Scaramouch, acrobat, aerialist, apple knocker, arriviste, ass,
   banana, bareback rider, be foolish, be stupid, blockhead,
   blunderer, blunderhead, boor, born fool, botcher, bounder,
   bourgeois, bucolic, buffo, buffoon, bumbler, bumpkin, bungler,
   burlesquer, cad, caper, caricaturist, churl, circus artist, clod,
   clodhopper, clodknocker, clot, clown around, comedian, comic,
   country bumpkin, cut up, cutup, dolt, doodle, egregious ass,
   epicier, epigrammatist, equestrian director, equilibrist, farceur,
   farmer, figure of fun, flier, fool, fool around, frivol, fumbler,
   funnyman, gag writer, gagman, gagster, galoot, gawk, get funny,
   go haywire, gowk, groundling, guttersnipe, harlequin, hayseed,
   hick, high-wire artist, hillbilly, hooligan, horse around,
   humorist, ignoramus, ill-bred fellow, invite ridicule, ironist,
   jack-pudding, jackass, jerk, jester, joker, jokesmith, jokester,
   juggler, klutz, lampooner, lion tamer, looby, lout, low fellow,
   lubber, lummox, lunatic, madcap, merry-andrew, milksop, mime,
   mooncalf, motley, motley fool, mountebank, mucker, mummer,
   nouveau riche, oaf, ox, parodist, parvenu, peasant, perfect fool,
   pickle-herring, play the buffoon, play the fool, prankster,
   provincial, punner, punster, quipster, reparteeist, ribald,
   ringmaster, rough, roughneck, rowdy, rube, ruffian, rustic,
   satirist, schmuck, slack-rope artist, slouch, slubberer,
   snake charmer, softhead, sop, stupid ass, sword swallower,
   tightrope walker, tomfool, trapeze artist, trifle, tumbler,
   upstart, vulgarian, vulgarist, wag, wagwit, wisecracker, wit,
   witling, yahoo, yokel, zany