noun
flat head; flat back of the head
Colloquial term for a head shape where the back is flat, often from sleeping position in infancy. Compare 短頭 (brachycephaly), which is a medical term.
See also: 短頭
赤ちゃんの絶壁頭が気になるなら、向き癖を直すといいよ。
If you're worried about your baby's flat head, it's good to correct their preferred sleeping position.
彼は自分の絶壁頭を気にしている。
He's self-conscious about his flat head.