The loose skin on the back of an animal's neck.
Scruff is often used when talking about picking up animals or when animals are in trouble.
Examples:
The mother cat carries her kittens by the scruff.
The dog's scruff is very loose.
He grabbed the kitten by the scruff.
The cat doesn't like being picked up by the scruff.
The lion caught the antelope by the scruff and carried it away.
The scruff of the dog's neck is very furry.