brawniness vs brawn

brawniness and brawn both are nouns.

Word	Noun	Adjective	Verb	Adverb
brawniness	Yes	No	No	No
brawn	Yes	No	No	No

As nouns, brawniness and brawn are synonyms defined as:

Other synonyms of brawniness include heftiness, muscle, muscularity, sinew.

brawniness (noun)	brawn (noun)
possessing muscular strength	possessing muscular strength