Thick Skin is the first Prayer that a player can use. It increases the user's Defence by 5% while activated. It cannot be used with any of the other prayers that provide a defence bonus, Chivalry and Piety included. This is the first prayer available to a player and also the first prayer to affect defence.
In 2000, Thick Skin was a Magic spell. It was one of the first spells ever added, but after a Prayer update in 2001 it was deleted and added to Prayer.