What word or phrase does your mother always say?
early 14c., from Old French burniss- present participle stem of burnir, metathesis of brunir "to shine, gleam, sparkle" (trans.), "to polish, make sparkle, make bright, shine," from brun "brown; polished," from a Germanic source (cf. Old High German brun, Old Norse brunn "bright, polished; brown;" see brown (adj.)). The connection to "brown" might be explained if the original objects in mind were wooden ones. Related: Burnished; burnishing.