Old English brun "dark, dusky," developing a definite color sense only 13c., from Proto-Germanic *brunaz (source also of Old Norse brunn, Danish brun, Old Frisian and Old High German brun, Dutch bruin, German braun), from PIE root *bher- (2) "bright; brown."
The Old English word also had a sense of "brightness, shining," preserved only in burnish. The Germanic word was adopted into Romanic (Middle Latin brunus, Italian and Spanish bruno, French brun). Brown sugar is from 1704. Brown Bess, slang name for old British Army flintlock musket, is first recorded 1785. Brown study "state of mental abstraction or meditation" is from 1530s; OED says the notion is "gloomy." Brown-paper "kind of coarse, stout, unbleached paper used for wrapping" is from 1650s.
c. 1300, "to become brown," from brown (adj.). From 1560s as "to make brown." Related: Browned; browning.