or Thumbscrew. An instrument of torture largely used by the Inquisition. The torture was compressing the thumb between two bars of iron, made to approach each other by means of a screw. Principal Carstairs was the last person put to this torture in Britain; he suffered for half an hour at Holyrood, by order of the Scotch Privy Council, to wring from him a confession of the secrets of the Argyll and Monmouth parties.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894