Source : Webster's Revised Unabridged Dictionary (1913)

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.

Imbecile \Im"be*cile\, n.
   One destitute of strength; esp., one of feeble mind.

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

Source : WordNet®

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