Meaning of ねくび in Japanese

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Definition of ねくび

ねくび(nekubi) 【寝首】

ねくびをかく(nekubiwokaku) 【寝首をかく ·寝首を掻く】

(exp, v5k) to cut off the head of a sleeping person; to assassinate someone while he sleeps
いつ寝首を掻かれるかわかったものではありません。
There's no telling when she'll stab you in the back.
to catch someone off his guard by setting traps