A cowlick is a tuft of hair that will not lie flat. A stubborn cowlick, flatten the cowlick, a cowlick at the crown.
It is caused by hair growing in a spiral from one point, so no amount of brushing will change its direction. The name says a cow licked the hair into place, which is exactly what it looks like.
1nouncowlickA tuft of hair that will not lie flat.
“No matter how much water he put on it, his cowlick kept sticking up at the back.”
cowlick is a tuft.
Cow plus lick, from the look of hair licked into place by a cow.
From cow + lick; as the hair may appear to have been licked into place by a cow.
Etymology from Wiktionary, CC BY-SA