Definition of brow noun from the Oxford Advanced American Dictionary

brow

noun
/braʊ/
 
 
1 (literary) the part of the face above the eyes and below the hair synonym foreheadThe nurse mopped his fevered brow.Her brow furrowed in concentration.2 [usually plural] = eyebrowOne dark brow rose in surprise.3 [usually singular] the top part of a hillThe path disappeared over the brow of the hill. see also highbrow, lowbrow, middlebrowIDIOM

knit your brow(s)

to move your eyebrows together, to show that you are thinking hard, feeling angry, etc.
synonym frownHis brows were knitted in a permanent frown.knit your browknit your brows