Arso > The word idiot may indeed come from the Greek equivalent, but when it’s used by Chazal it doesn’t mean idiot.
Greek idiots seems to be similar to Hebrew amei haaretz – citizens, but not being active in politics and possibly lacking manners. So kohen idiot was proper greek borrowing.
Moving to Latin, it was more on uneducated and lacking derech eretz, and French & English made idiots what they are today.
Btw do NOT google for kohen idiot, it brings up a non-kosher witness at a current trial, R’L