French word poilu comes from French poil (Bristle (on a brush). Hair (on the body).), French -u

Detailed word origin of poilu

Dictionary entry	Language	Definition
poil	French (fra)	Bristle (on a brush). Hair (on the body).
-u	French (fra)	Forming adjectives having the sense of ‘having quality of, being provided with’ (the root word).
poilu	French (fra)	(historical) poilu (French soldier during World War I) (dated, figurative) brave, courageous; energetic [late 19th c.]. (informal) hairy.

Words with the same origin as poilu