Horsehair is the long, strong hair from a horse’s mane and tail. People used to pack it inside chairs and sofas, because it springs back and lasts a long time; you may still find it in old furniture. The same hair is also used on violin bows. The word feels a little old when you talk about stuffing, because factories now more often use other fillings. It is hair from a horse, not a haircut, and not the hair on a person’s head.