Someone or something that is very irritating. "Royal" is used as an intensifier. Of course the client has more demands. Ugh, he is a royal pain. This project has turned into a royal pain—I doubt well get it done by the deadline.

Someone or something that causes a large or severe amount of frustration, annoyance, or aggravation. This new computer is proving to be a royal pain. It's been crashing at least once a day since I first started it up! These federal investigators have been royal pains. I know they have a job to do, but I wish they would just let us get on with our work.