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