cretin, Cretan

Cretin or cretin is an archaic medical term, meaning a person deformed by a thyroid deficiency--
Cretin was once a medical diagnosis, thought to be congenital, or possibly caused by a lack of sunlight.

Cretin is more commonly used as an offensive term for a stupid person--
No, you may not call your brother a cretin!

Cretan ( a proper noun, always capitalized ) means a person from Crete, an island in the Mediterranean, politically a part of Greece--
According to some legends, Zeus, king of the gods, was a Cretan.

Cretan may also describe something that comes from Crete--
He enjoys Cretan literature, and he can talk about it at length.

Now that you know that, you can say--
"He is a famous Cretan actor; don't behave like a cretin."