I still like the word butt bag, but yeah. I mean it has that name because it's literally a pack meant to rest on your butt.
But again, I guess maybe the UK has a different level of understanding for the word fanny than the US. In the US if you said 'fanny' people would be like, 'What are you, 5?' because it's got a connotation of being on the same level of saying the word 'heck'.