brawn vs brawniness

brawn and brawniness both are nouns.

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

As nouns, brawn and brawniness are synonyms defined as:

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

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