The Collaborative International Dictionary of English v.0.48:

Moron \Mo"ron\ (m[=o]"r[o^]n), n. [Gr. mw^ros foolish, stupid.]
   1. (Pedagogy) A mentally retarded person whose intellectual
      development proceeds normally up to about the eighth year
      of age and is then arrested so that there is little or no
      further development; an adult having the mental
      development of an 8-to-12-year old. A moron is considered
      capable of doing routine work under supervision.
      [Webster 1913 Suppl. +PJC]

   2. Hence, a very stupid person. [informal, derogatory]
      [PJC] -- mo*ron"ic adj. -- mo*ron"ic*al*ly adv. --
      mo"ron*ism n. -- mo*ron"i*ty n.

The Collaborative International Dictionary of English v.0.48:

Moron \Mo*ron"\ (m[-o]*r[=o]n"), n.; Sp. pl. Morones. [Sp.]
   An inferior olive size having a woody pulp and a large
   clingstone pit, growing in the mountainous and high-valley
   districts around the city of Moron, in Spain.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

moron
    n 1: a person of subnormal intelligence [syn: idiot,
         imbecile, cretin, moron, changeling, half-wit,
         retard]
    2: a city in Argentina, to the west of Buenos Aires

Moby Thesaurus II by Grady Ward, 1.0:

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