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.
By the late 18th century, peak was being applied to the hairline. From Courtney’s 1795 Isabinda of Bellefield:
Her forehead, rather low than high, on which her hair descends in a little peak, which looks exceedingly pretty.
The association of a peaked hood with widows goes back to the beginning of the 18th century. From Joseph Addison’s 1706 Rosamond:
Widow Trusty, why so Fine? Why dost thou thus in Colours shine? Thou should’st thy husband’s death bewail In Sable vesture, Peak and Veil.
And finally, the term widow’s peak appears in 1849 in Longfellow’s Kavanagh:
She had on her forehead what is sometimes denominated a “widow’s peak,"—that is to say, her hair grew down to a point in the middle.
(Source: Oxford English Dictionary, 2nd & New Editions)
Copyright 1997-2013, by David Wilton