Source : Webster's Revised Unabridged Dictionary (1913)

Thumbscrew \Thumb"screw`\, n.
   1. A screw having a flat-sided or knurled head, so that it
      may be turned by the thumb and forefinger.

   2. An old instrument of torture for compressing the thumb by
      a screw; a thumbkin.

Source : WordNet®

thumbscrew
     n 1: instrument of torture that crushes the thumb
     2: screw designed to be turned with the thumb and fingers