head of a sleeping person
Refers specifically to the head of someone who is asleep. Most often encountered in the idiom 寝首をかく (to cut off a sleeping person's head; to betray someone off guard).
He wouldn't do anything that would get him stabbed in the back.
油断していると、寝首をかかれるぞ。
If you let your guard down, you'll be betrayed.
Compound of 寝 (ne, 'sleep') and 首 (kubi, 'neck/head'). The exact historical derivation is uncertain, but the word is conventionally associated with the image of a sleeping person's head.