寝ね首くびをかく

expression, godan verb (-ku)

cut off a sleeping person's head; assassinate someone in their sleep

Literal meaning: to kill a sleeping person by cutting their neck. Often used in historical or dramatic contexts.

• 戦せん国ごく時じ代だいには、寝ね首くびをかくような卑ひ怯きょうな手ても使つかわれた。

  In the Warring States period, cowardly tactics like cutting off a sleeping person's head were also used.

expression, godan verb (-ku)

catch someone off guard by setting traps; stab someone in the back

Figurative extension: to betray or ambush someone who trusts you, often through scheming or traps. Implies underhandedness.

• 彼かれは親しん友ゆうに寝ね首くびをかかれて、会かい社しゃを追おわれた。

  He was stabbed in the back by his close friend and forced out of the company.

• 油ゆ断だんしていると、ライバルに寝ね首くびをかかれるぞ。

  If you let your guard down, your rivals will catch you off guard.