An especially irritating, aggravating, or obnoxious person, thing, or situation. You know, Jack, you may be my friend, but you can be a real pain in the butt sometimes! This calculus homework is a real pain in the butt. It's not that I don't understand it, it's just so tedious!

(used before a noun) Very irritating, aggravating, or obnoxious. I've still got a few pain-in-the-butt jobs to do around the ranch before I can call it quits for the day. We're having Mary over for dinner tomorrow night. I just hope her pain-in-the-butt husband doesn't come along as well.