Scruff is the loose skin at the back of an animal's neck. Mother cats carry their kittens by gently gripping this scruff in their mouths. The kittens dangle without getting hurt because this skin is tough and has fewer nerve endings than some other areas. Veterinarians sometimes hold cats by the scruff during examinations, though they're careful to support the cat's weight with their other hand.

People sometimes use it when describing how someone roughly grabbed another person: “The security guard caught the shoplifter by the scruff of his neck.” This suggests a firm, no-nonsense grip on someone's collar or the back of their shirt.

You might also hear someone described as scruffy, which means messy or unkempt, like a dog with matted fur or a person wearing rumpled clothes. A scruffy appearance suggests someone hasn't been paying much attention to grooming or neatness.