imbecile

n. 1 colloq. Stupid person. 2 person with a mental age of about five. adj. Mentally weak; stupid, idiotic.  imbecilic adj. Imbecility n. (pl. -ies). [french from latin]

a. Destitute of strength, whether of body or mind; feeble; impotent; esp., mentally wea; feeble-minded; as, hospitals for the imbecile and insane.