The Collaborative International Dictionary of English v.0.48:

Imbecile \Im"be*cile\, n.
   1. One destitute of strength; esp., one of feeble mind; --
      sometimes used as a pejorative term.
      [1913 Webster]

   2. (Psychology) A person with a degree of mental retardation
      between that of an idiot and a moron; in a former
      classification of mentally retarded person, it applied to
      a person with an adult mental age of from four to eith
      years, and an I.Q. of from 26 to 50.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Imbecile \Im"be*cile\, v. t.
   To weaken; to make imbecile; as, to imbecile men's courage.
   [Obs.] --Jer. Taylor.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Imbecile \Im"be*cile\, a. [L. imbecillis, and imbecillus; of
   unknown origin: cf. F. imb['e]cile.]
   Destitute of strength, whether of body or mind; feeble;
   impotent; esp., mentally wea; feeble-minded; as, hospitals
   for the imbecile and insane.

   Syn: Weak; feeble; feeble-minded; idiotic.
        [1913 Webster]

WordNet (r) 3.0 (2006):

imbecile
    adj 1: having a mental age of three to seven years [syn:
           imbecile, imbecilic, idiotic]
    n 1: a person of subnormal intelligence [syn: idiot,
         imbecile, cretin, moron, changeling, half-wit,
         retard]