Did you not find an answer yourself; i.e. ‘asshole’? Most other anatomical inserts also apply, at least the non-gendered ones. Does not offend any particular group, except perhaps those with a colostomy. Still an insult. The remainder are generally insulting because they imply you’re so X that you must be a member of group X. I.e. calling someone brain-dead, an idiot, a moron, slow, the French word for slow etc. You can’t call someone stupid without calling them stupid, even in an indirect way.