The Collaborative International Dictionary of English v.0.48:

Cretin \Cre"tin\ (kr[=e]"t[i^]n), n. [F. cr['e]tin; of uncertain
   origin.]
   One afflicted with cretinism.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cretin
    n 1: a person of subnormal intelligence [syn: idiot,
         imbecile, cretin, moron, changeling, half-wit,
         retard]

The Jargon File (version 4.4.7, 29 Dec 2003):

cretin
 /kret'in/, /kree´tn/, n.

    Congenital loser; an obnoxious person; someone who can't do anything
    right. It has been observed that many American hackers tend to favor the
    British pronunciation /kret'in/ over standard American /kree'tn/; it is
    thought this may be due to the insidious phonetic influence of Monty
    Python's Flying Circus.