Informal. a dunce; blockhead; dolt: Don't stand there like an imbecile. Open the door!.
Psychology. (no longer in technical use; now considered offensive) a person of the second order in a former and discarded classification of mental retardation, above the level of idiocy, having a mental age of seven or eight years and an intelligence quotient of 25 to 50.
Informal. stupid; silly; absurd.
Usually Offensive. showing mental feebleness or incapacity.
Archaic. weak or feeble.
Origin of imbecile
1540–50; earlier imbecill < Latinimbēcillus weak; -ile replacing -ill by confusion with suffix -ile
Related formsim·be·cil·ic, adjectiveim·be·cile·ly, adverb
1540s, imbecille "weak, feeble" (especially in reference to the body), from Middle French imbecile (15c.), from Latin imbecillus "weak, feeble" (see imbecility). Sense shifted to mental weakness from mid-18c. As a noun, "feeble-minded person," it is attested from 1802. Traditionally an adult with a mental age of roughly 6 to 9 (above an idiot but beneath a moron).
A person of moderate to severe mental retardation having a mental age of from three to seven years and generally being capable of some degree of communication and performance of simple tasks under supervision. The term belongs to a classification system no longer in use and is now considered offensive.