The Collaborative International Dictionary of English v.0.48:

moronic \moronic\ adj.
   having a mental age of between eight and twelve years; also
   used as a derogatory term.
   [WordNet 1.5]

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.

WordNet (r) 3.0 (2006):

moronic
    adj 1: having a mental age of between eight and twelve years