> But postmortem is literally “post death” in Latin.

Sure, that's true. It's not really relevant to people who don't speak Latin.

> It’s synonymous with autopsy!

This isn't true. I have no idea how you got here. In English an autopsy is a surgical procedure performed on a corpse for the purpose of identifying the cause of death. In particular, it's a noun. "Post-mortem" in the etymologically literal "after death" sense is an adverb or adjective identifying whatever it is as taking place chronologically after some contextually-specified death.

But, you seem to put some weight on the etymology of a word independently of its current meaning. In that case autopsy "literally means" to witness something personally, as opposed to hearing about it from someone else. (It's "self-eye" in Greek just as post mortem is "after death" in Latin.) Somehow I doubt that's what you had in mind...?