A Happy Sock is a catnip toy made from a sock. You can see several examples of Happy Socks in this photo.
Happy Socks are super simple to make: just take a sock and stuff it with a fistful of polyfill and a big pinch of catnip. Tie a knot in the ankle and you've made a Happy Sock!
Happy Socks can be made from almost any kind of sock. And cats love Happy Sock catnip toys because they're big, fluffy and durable.