obesity

Obesity comes from French obésité, which traces to Latin obēsitās, from obēsus meaning 'fat.' By surface analysis in English, it's simply obese + -ity.

The word entered English as a direct borrowing of a fully formed noun, so its sense has stayed tied to its Latin root's meaning of fatness rather than drifting far from it.

It shares its root with the adjective 'obese,' which comes from the same Latin word obēsus — the noun form built on the adjective.