imbecile (adj.) Look up imbecile at Dictionary.com
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. (compare frail, which in provincial English also could mean "mentally weak"). 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).