Definition & Meaning of "a pain in the ass"in English

What is the origin of the idiom "a pain in the ass" and when to use it?

A pain in the ass is an idiomatic expression that appeared in mid-20th-century American English to describe a person or thing that causes ongoing annoyance, trouble, or inconvenience. The phrase uses the body part metaphorically: pain in a sensitive area equals constant irritation that's hard to ignore. It isn't sexual in meaning; it's about persistence and frustration. While clearly informal and crude, it's widely used in everyday speech and can range from mildly rude to strongly annoyed depending on tone and context.