寝ね首くび

noun

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).

• 彼かれは寝ね首くびをかかれるような真似まねはしない。

  He wouldn't do anything that would get him stabbed in the back.

• 油ゆ断だんしていると、寝ね首くびをかかれるぞ。

  If you let your guard down, you'll be betrayed.