hair on the back of one's head
Refers specifically to the hair at the back of the head. Most often encountered in the idiomatic expression 後ろ髪を引かれる (to feel reluctant to leave; literally 'to have one's back hair pulled').
See also: 後ろ髪を引かれる
She left her hometown with a heavy heart, as if being pulled back by the hair.
I left the party feeling reluctant, as if my back hair were being pulled.