A callus (or callosity) is a toughened area of skin which has become relatively thick and hard in response to repeated friction, pressure, or other irritation.Rubbing that is too frequent or forceful will cause blisters rather than allow calluses to form.ormally, a callus will form on any part of the skin exposed to friction over a long period of time. For example, people often develop calluses on the middle finger of their dominant hand due to writing with a pen or pencil.