We live in a world populated by idiots, morons, and imbeciles. So much so that it seems a wonder the world doesn't grind to a halt. From time to time the question comes up about which is worse; an idiot, a moron, or an imbecile. We certainly use them often enough that correct usage matters. Very little remains of my years of study of French — beyond a deep appreciation for the art of kissing and the role of butter in brown sauces — but I do remember the insults "Espece d'idiot!" and "Imbecile!". Since proper usage matters, here's the inside scoop.
idiot — A person of profound mental retardation having a mental age below three years and generally being unable to learn connected speech or guard against common dangers.American Heritage Stedman's Medical Dictionary
imbecile — 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.American Heritage Stedman's Medical Dictionary
moron — A person of mild mental retardation having a mental age of from 7 to 12 years and generally having communication and social skills enabling some degree of academic or vocational education.American Heritage Stedman's Medical Dictionary
Imbecile comes from Old French meaning weak or feeble, which is derived from the Latin "imbcillus" meaning staff or rod. (Perhaps because of using a cane or staff due to physical weakness.) Moron is dervied from Greek "mron" meaning stupid or foolish. Idiot is derived from the Old French "idiote" which comes from the Latin "idita", which, in turn, comes from the Greek "idits" meaning private person. (Idios versus koinos for those of you who remember your Greek.)
So there you have it: idiot (mental age < 3 years), imbecile (3 years < mental age < 7 years), and moron (7 years < mental age < 12 years). The relative positions can be remembered using alphabetical order. These terms must never, of course, be applied to developmentally disabled people, with one exception: the current President of the United States of America. Oh yeah, and that moron who picked the wrong running mate and lost the election.