pain in the arse

[Carolina]

Where does the expression 'pain in the arse' comes from and why?

[andy hughes]

I believe the original expression - 'a pain in the neck' derives from formal dress from an earlier age, when stiff starched collars and brass collar studs woulc cause a literal pain in the neck if worn for a long period of time. The expression is associated with the need to dress up, and wear said formal attire for an uncomfortable time, thus giving rise to 'a pain in the neck'. I guess the more blunt version is simply an adaptation of the phrase.