A person or task that is irritating, challenging, or annoying, usually more so than it has to be.
I am a medic and every two years I have to get recertified. It's an unnecessary pain in the ass, because I have to file about 10 million papers and every two years they change it, and it's pointless anyways because I'm obviously not going to suddenly forget how to do my job.
Any person who thinks that Zuko and Katara should have ended up together.
Person A: "OMG Zutara all the way!!!!"
Person B: "You're such a pain in the ass"
My nephew
Someone who annoys the shit out of you for no reason
My nephew is a pain in the ass
Something really annoying that really pisses you off.
Wake up everyday at 6 a.m is a real pain in the ass
A task, often bureaucratic, that is more difficult than it needs to be.
Now I need three forms of ID and a copy of my electric bill just to renew my driver's license? What a pain in the ass!
My headlight fuse burned out. Changing it was a real pain in the ass because I had to take apart half the dashboard to get to the fuse panel.
Thing that bothers you very much, but it's hard or inappropriate to put them on the table to talk about because of the bother-maker's identity.
My boss always leaves no stone untured to make fun of me in order to cheer him up, so he is a real pain in the ass.