thumbscrew

noun

1. A screw that is designed to be turned with the thumb and fingers, thus requiring no screwdriver or spanner (wrench).

   “Very interested, Dr Grimsby watched the saw follow the fine pencil line on the white pearwood as Peter dexterously twirled the veneers on the fretwork table, timing the movement of both hands precisely to put no strain on either the saw or the wood. Nor did he acknowledge their presence till he had finished a section of the pattern, gingerly released the saw from its upper thumbscrew, and drawn it out of the wood.”

2. An instrument of torture used to crush the fingers.

   “To ensure good behavior, the slaveholder relies on the whip; […] to imbrute and destroy his manhood, he relies on the whip, the chain, the gag, the thumb-screw, the pillory, the bowie-knife, the pistol, and the blood-hound.”

3. (figuratively)A weakness that can be taken advantage of.

   “Law 33: Discover Each Man's Thumbscrew ― Robert Greene, The 48 Laws of Power”