It's not offensive (unless used as a designation for a person) and not slang but vulgar it is, not quite aslike жопа
оr worse, but enough to be avoided in a polite conversation.
Instead some literary/medical/anatomical terms are preferred or naming it with euphemisms.
It's not tagged as vulgar in the dictionary unlike жопа
https://ru.wiktionary.org/wiki/задница
But none of the usage examples can be considered polite or decent