hair at the nape of the neck; scruff of the neck
Refers to the hair growing at the back of the neck, often mentioned in contexts of grabbing or cutting.
He was dragged by the scruff of his neck.
The hair on my nape has grown long, so I'm going to get a haircut.
Standard kanji spelling for the nape hair.
Kana spelling is sometimes used, especially in casual or explanatory contexts.
Compound of 襟 (eri, 'collar, neck') and 髪 (kami, 'hair'), with rendaku voicing of 髪 to がみ. The exact historical derivation is uncertain, but the word straightforwardly describes the hair at the nape.