Expressions (phrases, clauses, etc.), Godan verb with 'ku' ending
to cut off the head of a sleeping person; to assassinate someone while they sleep
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 their guard by setting traps
Also written 寝首を掻く【ねくびをかく】