thumbscrew

/TH AH1 M S K R UW1/
noun
  1. 1

    A screw that can be turned with the thumb and fingers.

  2. 2

    An instrument of torture used to crush the fingers.

  3. 3

    A weakness that can be taken advantage of.

    โ€œLaw 33: Discover Each Man's Thumbscrew โ€• Robert Greene, The 48 Laws of Powerโ€

Synonyms

Source: https://en.wiktionary.org/wiki/thumbscrew