A goth or emo way of saying 'look what the cat dragged in'. Cats are known for bringing dead mice/rats/birds and other vermin they may have found or killed in to their owner's home. Upon meeting someone who is disliked, by using this phrase you imply that they are the vermin mentioned above.
A statement in order to convey that whomever has said this has absolutely no control of the situation. Also used in conjunction with Monkey Shit Fights, SNAFU, ATFU, FUBAR.