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.
John - "stfu you idiom"
n00b - "iDiom? FuCK YOu, YouR AN iDioT"
John - " *Shakes head* "
American (taking out a Battleship game): Mind if I sink a steamer?
Mexican (taxi driver): Nope...hey what's that smell?