early 14c., browes, brues "brow, forehead, eyebrow," earlier brouwes (c.1300), bruwen (c.1200), from Old English bru, probably originally "eyebrow," but extended to "eyelash," then "eyelid" by association of the hair of the eyebrow with the hair of the eyelid, the eyebrows then becoming Old English oferbrua "overbrows" (early Middle English uvere breyhes or briges aboue þe eiges).
The general word for "eyebrow" in Middle English was brew, breowen (c.1200), from Old English bræw (West Saxon), *brew (Anglian), from Proto-Germanic *bræwi- "blinker, twinkler" (cf. Old Frisian bre, Old Saxon brawa, Middle Dutch brauwe "eyelid," Old High German brawa"eyebrow," Old Norse bra "eyebrow," Gothic brahw "twinkle, blink," in phrase in brahwa augins "in the twinkling of an eye").
Old English bru is from Proto-Germanic *brus- "eyebrow" (cf. Old Norse brun), from PIE *bhru- "eyebrow" (cf. Sanskrit bhrus "eyebrow," Greek ophrys, Old Church Slavonic bruvi, Lithuanian bruvis "brow," Old Irish bru "edge"). The -n- in the Old Norse (brun) and German (braune) forms of the word are from a genitive plural inflection.
Words for "eyelid," "eyelash," and "eyebrow" changed about maddeningly in Old and Middle English (and in all the West Germanic languages). By 1530s, brow had been given an extended sense of "forehead," especially with reference to movements and expressions that showed emotion or attitude.