brow (n.) c. 1300, broue, plural broues, brouen, "arch of hair over the eye," also extended to the prominent ridge over the eye (early 14c.), from Old English bru (plural brua), which probably originally meant "eyebrow" (but also was used in the sense of "eyelash"), from Proto-Germanic *brus- "eyebrow" (source also of Old Norse brun), from PIE *bhru- "eyebrow" (source also of 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.

Sources:

• A Dictionary of Selected Synonyms in the Principal Indo-European Languages, University of Chicago (By: Carl Darling Buck) 1988.
• Etymological Dictionary of Latin and the other Italic Languages (By: Michiel de Vaan) 2008.
• The American Heritage Dictionary of Indo-European Roots (By: Calvert Watkins) 2000.
• Noun Declension in Indo-European (Sindhueuropaia Deklination Nomnes) By: Carlos Quiles.
• Online Etymology Dictionary (By: Douglas Harper)