Pain in the Ass; Pain in the Butt; Pain in the Neck

What does the idiom “Pain in the Ass; Pain in the Butt; Pain in the Neck” mean?

Definition:

An ongoing annoyance, an irritant

For example:

“Edward can be a pain in the neck sometimes, but he’s an excellent accountant.”

Notes:

“Pain in the ass/arse” is slightly obscene.