The Collaborative International Dictionary of English v.0.48:

Tomfool \Tom"fool`\ (t[o^]m"f[=oo]l`), n. [Tom (see Tomboy) +
   fool.]
   A great fool; a trifler.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tomfool
    n 1: a person who lacks good judgment [syn: fool, sap,
         saphead, muggins, tomfool]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "tomfool":
   absurd, ass, balmy, born fool, buffoon, clown, crazy, donkey,
   doodle, egregious ass, fantastic, figure of fun, fool, harebrained,
   idiot, ignoramus, imbecile, insane, jackass, jerk, loony, lunatic,
   milksop, mooncalf, nincompoop, ninny, perfect fool, preposterous,
   schmuck, silly, softhead, sop, stupid ass, zany