The Collaborative International Dictionary of English v.0.48:

Simpleton \Sim"ple*ton\, n. [Cf. F. simplet, It. semplicione.]
   A person of weak intellect; a silly person.
   [1913 Webster]

WordNet (r) 3.0 (2006):

simpleton
    n 1: a person lacking intelligence or common sense [syn:
         simpleton, simple]

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "simpleton":
   ament, born fool, bungler, clot, congenital idiot, cretin,
   defective, dullard, dullhead, dumbbell, dummkopf, dummy, golem,
   half-wit, idiot, ignoramus, imbecile, juggins, mongoloid idiot,
   moron, natural, natural idiot, natural-born fool, simp, stupid,
   zany