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.
adj.
also im·be·cil·ic (ĭm'bə-sĭl'ĭk)
Stupid; silly.
Well below par in mental acumen.
[From obsolete French imbécille, weak, feeble, from Old French, from Latin imbēcillus : in-, not; see in-1 + possibly bacillum, staff, diminutive of baculum, rod; see bak- in Indo-European roots.] im'be·cile·ly adv.