I'd agree with Lucy's answer, but there's one more option. Sometimes in very informal spoken English we add 'my ass' onto the end of a comment to say that we don't believe it. A: He's the best teacher in the world B: The best teacher in the world, my ass! (B doesn't agree with A's statement) I'm not sure if this could apply to your example, as I would need to see more of the context, but it is one possibility!

This isn't great English or American - perhaps you misheard it? If you want to translate it literally - see below. But actually it is't a phrase anyone would use. "He is literally my ass" = he is literally by backside 'ass' is a slang word in the US for bottom/backside Ass in English means a type of donkey - Go figure as they say.

I think you are missing this - a pain in --- my ass. He is literally a pain in my ass. It's a common slang style phrase you read and see in videos and movies, at least in the USA anyway...