diabetes mellitus just means sweet piss.

Not quite. It more means water passes right though you and tastes sweet. Compare that to diabetes insipidus where you're pissing out massive amounts of undiluted pee with no taste whatsoever. It's also another fucking annoying example of mixing Greek and Latin, with diabetes coming from Greek meaning to pass through/siphon and mellitus using a Latin suffix. But that's pretty much medicine naming in general.