early 14c., "to polish by friction," from Old French burniss- present participle stem of burnir, metathesis of brunir "to shine, gleam, sparkle" (trans.), also "to polish, make sparkle, make bright, shine," from brun "brown; polished," from Frankish or some other Germanic source (compare Old High German brun, Old Norse brunn "bright, polished; brown"), from Proto-Germanic *brunaz (from PIE root *bher- (2) "bright; brown"). The connection to "brown" might be explained if the original objects in mind were wooden ones. Meaning "cause to glow, clean (something) until it shines" is from late 14c. Related: Burnished; burnishing.