Definiția și Sensul lui "a pain in the (ass|butt)"

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

The idiom "a pain in the ass" is an informal and somewhat vulgar expression, believed to have emerged in the early 20th century as a colloquial expression in American English and has become a widely used phrase in informal conversations.. It is used to describe someone or something that is extremely annoying, troublesome, or difficult to deal with.