"You're such an ass pain."

Nope. It's always "You're such a pain in the ass."

When will people learn that there is no need to expend the extra energy to say "in the" if you simply invert "pain" and "ass"? Not only is it easier, but it also seems to do well in getting the attention of the insulted one (or any eavesdroppers). Although they will strive to correct you, it is not you who needs the correcting. They are the ones wasting away their linguistic abilities with extraneous syllables.