Other forms: scruffs
The scruff is the loose skin at the back of the neck. A cat carries her kittens by it.
The phrase by the scruff of the neck is where the word lives, and it means gripping somebody there and moving them, which is a real thing done to animals and a metaphor for taking control of a situation. There is a separate British informal noun, since a scruff is an untidily dressed person, and the adjective scruffy is far commoner than either.
The scruff of the neck belongs to a horse. The eighteenth-century form was scuft, related to Dutch schoft, the withers of a horse, and to Old Norse skopt, the hair of the head.
See scurf.
Etymology from Wiktionary, CC BY-SA