I'm not a 'bum' man, myself, and 'ass' is something you say about someone. 'Butt' is...okay, but 'buttocks' always reminds me of a couple of jokes. Some writer for the old Star Distributors was ennamored of 'cakes' and 'buttcakes'. Gag me.

It's not totally satisfactory, but I tend toward 'rear'.

And 'knickers' causes snickers.