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]

Moby Thesaurus II by Grady Ward, 1.0:

160 Moby Thesaurus words for "imbecile":
   ament, amputee, anemic, apish, arrested, asinine, ass, asthenic,
   babbling, backward, batty, befooled, beguiled, besotted,
   blithering, bloodless, born fool, brainless, buffoonish, burbling,
   chicken, clot, cockeyed, congenital idiot, cowardly, crackbrained,
   cracked, crazy, credulous, cretin, cretinistic, cretinous, cripple,
   daffy, daft, dazed, debilitated, defective, deformity, dim-witted,
   dithering, dizzy, donkey, doting, driveling, drooling, drooping,
   droopy, dull, dumb, effete, etiolated, faint, faintish, fatuitous,
   fatuous, feeble, feebleminded, flabby, flaccid, flaky, floppy,
   fond, fool, foolheaded, foolish, fuddled, futile, gaga, golem,
   gone, goofy, gulled, gutless, half-baked, half-wit, half-witted,
   handicapped person, idiot, idiotic, imbecilic, impotent, inane,
   incapable, inept, infatuated, insane, jackass, jerk, juggins,
   kooky, languid, languorous, limber, limp, listless, loony,
   lustless, mad, marrowless, maudlin, maundering, mentally defective,
   mentally deficient, mentally handicapped, mentally retarded,
   mongoloid, mongoloid idiot, moron, moronic, natural, natural idiot,
   natural-born fool, nerveless, nincompoop, ninny, not all there,
   nutty, paralytic, paraplegic, pithless, pooped, powerless,
   quadriplegic, retarded, rubbery, sapless, sappy, screwy, senseless,
   sentimental, silly, simp, simple, simpleminded, simpleton,
   simpletonian, sinewless, slack, slobbering, slow, slow-witted,
   soft, spineless, strengthless, stupid, subnormal, the crippled,
   the handicapped, thoughtless, tomfool, unhardened, unnerved,
   unstrung, wacky, weak, weakly, wet, witless, zany