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