Pain in the blank

Pain in the sitter
Hello all,
I would like to know the meaning of this british phrase. Anyone care to enlight me ?
Thx,
Jamie

It's the same as PITA. Sitter = what you sit with; aka bum, kiester, buttocks, bottom, derriere, gluteus maximus, ...

It's a minor point, but none of the dictionaries at my disposal allowed me to look up "kiester." They all insisted on "keister," prounounced kiester. It originally meant a suitcase or sachel, sometimes a sample case. SS