Meaning of 寝首 in Japanese

Definition of 寝首

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