The word "arses" is a vulgar slang term used to refer to a person's backside or buttocks. If you want to replace this term with more socially acceptable synonyms, there are many words you can use. Some common alternatives include "buttocks," "behind," "rear end," "derriere," "posterior," "bum," "backside," "bottom," "rump," and "glutes." Many of these terms are more formal than "arses," but they are still commonly used in casual conversation. Ultimately, the synonym you choose will depend on the level of formality you want to convey.