For the longest time, I thought the pejorative pronounced cree-tuhn simply meant a bad dude. After dabbling in the classics in college, I thought the word was derived from Cretan, people from Crete. You know, the liar paradox (that even made it to the New Testament):
Epimenides was a Cretan who made one immortal statement: "All Cretans are liars."
The Cretans, always liars, evil beasts, idle bellies!
However, that is only half the story. While cretan has become a insult for liars and bad dudes in general (no offense to the people of Crete, who undoubtedly run the spectrum of truth-telling and personality), the word cretin has become an insult for stupidity. As with many modern terms of insult, cretin has roots in now-archaic scientific terminology:
Cretin is the oldest and comes from a dialectal French word for Christian. The implication was that people with significant intellectual or developmental disabilities were "still human" (or "still Christian") and deserved to be treated with basic human dignity. This term has not been used in any serious or scientific endeavor since the middle of the 20th century and is now always considered a term of abuse: notably, in the 1964 movie Becket, King Henry II calls his son and heir a "cretin." "Cretinism" is also used as an obsolescent term to refer to the condition of congenital hypothyroidism, in which there is some degree of mental retardation.
The etymology of the word cretin is not known with certainty. Several hypotheses have been proposed. The most common derivation provided in English dictionaries is from the Alpine French dialect pronunciation of the word Chrétien - (a) Christian, which functioned as a form of greeting in those parts. According to the Oxford English Dictionary, the translation of the Latin term into "human creature" implies that the label "Christian" is a reminder of the humanity of the afflicted, in contrast to brute beasts. Other sources have suggested "Christian" refers to the "Christ-like" inability of such a person to commit sin, because of an incapacity to distinguish right from wrong.
Yes, of course I'll have to build a political straw man and defeat it with snarkiness: Why do liar cretans, stupid cretins, and quasi-Christians sound familiar? Now, I'm sure a decent argument can be made by Republicans against Democrats using the same above logic (spin is spin, after all) - but the past 7 1/2 years of a Republican administration has kinda-sorta tipped the scales against the GOP.
In any case, the McCain-Palin ticket has not yet made a strong enough case that their GOP policies would be drastically different than Bush-Cheney's GOP policies:
What's the difference between a cretan and a cretin? One vowel, duh. But if you said "Lipstick," then you are sexist, my friend.