From WordNet (r) 3.0 (2006) :
pain in the ass
n 1: something or someone that causes trouble; a source of
unhappiness; "washing dishes was a nuisance before we got a
dish washer"; "a bit of a bother"; "he's not a friend, he's
an infliction" [syn: annoyance, bother, botheration,
pain, infliction, pain in the neck, pain in the
ass]