Meaning/Usage: A source of trouble or annoyance; used to indicate that something bothers you very much. "Ass" can be seen as profanity, so another way of saying this would be "Pain in the butt" or "Pain in the neck."(麻烦或烦恼的来源；用来表示某事使你烦恼。“屁股”可以看作是亵渎神灵，所以另一种说法是“屁股痛”或“脖子痛”。)

Explanation: When something gives you trouble and bothers you a lot, it gives you pain. To look for more colorful ways to express this, people started saying "pain in the neck." Another variation came out when people started to say "Pain in the rear end." This later became "Pain in the butt" or "Pain in the ass."

"What a pain in the ass."
"I had a customer who kept asking easy questions. It was a total pain in the ass."
"I have a meeting at 7:00 in the morning. It's a pain in the ass waking up that early."
"Having to deal with traffic every day is a pain in the ass."

A: "I have to go to the post office."
B: "I thought you went yesterday?"
A: "They made a mistake on the postage so the package was returned."
B: "What a pain in the ass."

Other Common Sentences

"Going to get a health check up is a pain in the neck."
"I hate mowing the lawn. It's a real drag."
"I have to pick up my brother at the airport every week. It's a serious burden because I'm busy these days."