—n.
1 a person of abnormally weak intellect, esp. an adult with a mental age of about five.
2 colloq. a stupid person.
—adj. mentally weak; stupid, idiotic.
Derivatives:
imbecilely adv. imbecilic adj. imbecility n. (pl. -ies).
Etymology: F imbeacutecil(l)e f. L imbecillus (as IN-(1), baculum stick) orig. in sense 'without supporting staff'
Useful english dictionary. 2012.