Backside - это не спина: https://www.lingvolive.com/ru-ru/translate/en-ru/backside
Такой же словарь c примерами и голосовым произношением есть и в приложении (строка «ПЕРЕВЕСТИ» вверху или значок ЛУПА 🔍 внизу).
Спина - это BACK: https://www.lingvolive.com/ru-ru/translate/ru-en/спина

A pain in the backside is a person - A PAIN IN THE ASS - someone that when you see them coming, you say "Oh God, not her/him!!" For example, someone who comes over and just talks and talks and doesn't let you do anything. Or someone who calls you five times a day asking for favors and things. Someone who you want to avoid at all costs because they annoy the hell out of you. I'm sure there's a Russian word for this (if not several) - some of them use this site!)))

A PAIN IN THE BACKSIDE/ASS can also be a task that's complicated, boring, tedious, etc.: "Moving is a pain in the ass!"

Ахад, please remove that translation - it's incorrect