Wordorigins.org

A widow’s peak is the sharp angle of the hairline on the forehead of some people. It’s so called because it resembles the peak of a hood traditionally worn by women in mourning. The use of peak referring to a point in the cloth covering the forehead dates to at least 1509 when it appears in Alexander Barclay’s The Shyp of Folys:

And ye Jentyl wymen whome this lewde vice doth blynde Lased on the backe: your peakes set a loft.