The thumbscrew is an instrument of torture which was used in medieval Europe, notably by the Inquisition. It is a simple vice, sometimes with protruding studs on the interior surfaces. The torture victim's thumbs or fingers were placed in the vise and slowly crushed. The thumbscrew was also applied to crush prisoners' toes, while larger, heavier devices based on the same design principle were applied to destroy knees and elbows .

As well as compression of fingers and toes, the opposite concept of distension was also employed in torture chambers. Tearing the nails out of the fingers and toes as slowly as possible was a particularly common method of torture. The most straightforward means employed a pliers—often heated red-hot—to tear out the nails at the root. However, the nail extraction could be accomplished even more slowly and cruelly by first driving wooden wedges, needles, or splinters of wood, metal, or bone under the nails to pry them loose. Splinters in particular were sometimes dipped in boiling sulfur to make the torture even more savage.

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Thumbscrew is also used to refer to a screw with a tall head with ridged sides, intended to be tightened and loosened by hand. They usually have a slot for a flathead screwdriver as well.

Here are two links for more information on thumbscrews as well as a picture of how they look. [1][2]