Idiom
a pain in the ass
someone or something that is very annoying.He doesn't win cases — he just makes such a pain in the ass of himself that opponents give up.She acts like all the people she works with are a pain in the neck.
Usage notes: also used in the forms a pain in the butt or a pain in the backside:
Watering the lawn is always a pain in the butt.
I'm really not sure any of these are rude slang; arguably just informal--SL, 1/03