Old English brun "dark, dusky," developing a definite color sense only 13c., from Proto-Germanic *brunaz (cf. Old Norse brunn, Danish brun, Old Frisian and Old High German brun, Dutch bruin, German braun), from PIE *bher- (3) "shining, brown" (cf. Lithuanian beras "brown"), related to *bheros "dark animal" (cf. beaver, bear (n.), and Greek phrynos "toad," literally "the brown animal").
The Old English word also had a sense of "brightness, shining," preserved only in burnish. The Germanic word was adopted into Romanic (e.g. Middle Latin brunus, Italian and Spanish bruno, French brun). Brown Bess, slang name for old British Army flintlock musket, first recorded 1785.