I would never call you an imbecile when you are obviously an idiot.
Idiots, Morons, Imbeciles, Cretins
According to British legal statue, an idiot is an individual with an IQ of less than 20, an imbecile has an IQ of between 20 and 49, and a moron an IQ between 50 and 69. Cretins are specifically persons with a deformity or mental retardation caused by a thyroid deficiency; cretinism is now more commonly called hyperthyroidism. Idiot is derived from the Greek for "private person" (as in idiosyncratic); moron is from the Greek for "foolish"; imbecile is a construction from a Latin phrase meaning "without a stick"; cretin comes, via the French, from the word for "Christian" and implies a holy person--"God's fool," as it were.--excerpt [p.125] from The Uncyclopedia by Gideon Haigh