Wikipedia
Denailing—the forcible extraction of the nails from the fingers and/or toes—was a favorite method of medieval torture that remains in use today. Denailing is both highly efficient and extremely effective as a form of physical torture and, in modern use, causes limited physical injury: while brute-force tearing out can and does damage the cuticles, surgical extraction via scalpel and forceps without anesthesia does not.
In its simplest form, the torture is conducted by spread-eagling the prisoner to a tabletop—securing the hands by chains around the wrists and the bare feet by chains around the ankles—and using a metal forceps or pliers—often heated red-hot—to individually grasp each nail in turn and slowly pry it from the nail bed before tearing it off the finger or toe. A more painful variant used in medieval Spain was performed by introducing a sharp wedge of wood or metal between the flesh and each nail and slowly hammering the wedge under the nail until it was torn free.
Another cruel variant involved using rough skewers of wood or bone dipped in boiling sulfur. A number of such skewers were slowly driven into the flesh under the prisoner's toenails. Alternately, the skewer was dipped in boiling oil, which served a dual purpose of both burning the incredibly sensitive flesh and lubricating the needle so that the torturer could freely explore a wide surface area beneath the toenail. When enough skewers had been driven home to pry each nail loose from its bed, the nail was torn out at the root with a pair of pliers. It is also recorded that, in more recent times—particularly, during the Armenian genocide of the 1910s—phonograph needles were driven under fingernails to torture the prisoner before his nails were torn out with pliers.