expression, godan verb (-ru)
get a haircut; have one's hair cut
Literally 'to mow one's head'; a colloquial expression for getting a haircut, often implying a short or close cut. The subject is the person whose hair is cut, not the barber.
そろそろ頭を刈りに行かないと。
I need to go get a haircut soon.
夏になる前に頭を刈った。
I got my hair cut before summer.
The expression combines 頭 (head) with 刈る (to mow, to cut grass/hair), giving a vivid image of cutting hair like mowing a lawn. The exact origin is uncertain, but it is a natural extension of 刈る used for cutting hair.