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]