E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Thumbikins 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.