For slang use of imbecile, see Idiot.
Imbecile is a term for moderate to severe mental retardation, as well as for a type of criminal. It arises from the Latin word imbecillus, meaning weak, or weak-minded. "Imbecile" was once applied to people with an IQ of 26-50, between "moron" (IQ of 51-70) and "idiot" (IQ of 0-25).
The term was further refined into mental and moral imbecility. The concepts of "moral insanity," "moral idiocy," and "moral imbecility," led to the emerging field of eugenic criminology, which held that crime can be reduced by preventing "feeble-minded" people from reproducing.
"Imbecile" as a concrete classification was popularized by psychologist Henry H. Goddard and was used in 1927 by United States Supreme Court Justice Oliver Wendell Holmes, Jr. in his landmark ruling in the forced-sterilization case Buck v. Bell.
The term is closely associated with psychology, psychiatry, criminology, and eugenics. The term imbecile quickly passed into vernacular usage as a derogatory term, and fell out of professional use in the 20th century.