French word fessu comes from French fesse ((anatomy) buttock.), French -u

Detailed word origin of fessu

Dictionary entry	Language	Definition
fesse	French (fra)	(anatomy) buttock.
-u	French (fra)	Forming adjectives having the sense of ‘having quality of, being provided with’ (the root word).
fessu	French (fra)	Having fat buttocks.

Words with the same origin as fessu