noun
Refers specifically to the back of the neck, often used when grabbing or holding someone.
彼は弟の襟首をつかんで引きずった。
He grabbed his younger brother by the nape of the neck and dragged him.
猫の襟首をくわえて運ぶ親猫。
A mother cat carrying her kitten by the scruff of its neck.
うなじ is a more elegant or literary term for the nape, often used in poetic or aesthetic contexts, while 襟首 is more everyday and physical.
Compound of 襟 (eri, 'collar') and 首 (kubi, 'neck'). The reading えりくび is a straightforward combination of the two components.