Words or phrases that aren't meant to be taken literally For example, if Joe says “break a leg” he doesn’t really mean to break your leg. Break a leg actually means “good luck.”
A term used by people who are tired of calling n00bs "idiots".
The individual calls a n00b an "idiom" so that they'll get even more confused and react in a more n00bish way. It will confuse them until the n00b actually looks up the word in a dictionary to find the definition. Otherwise, they'll think idiom is another word for idiot.