From Longman Dictionary of Contemporary Englishthumbscrewthumb‧screw /ˈθʌmskruː/ noun [countable] SCJan object that was used in the past to punish people by crushing their thumbs
Examples from the Corpus
thumbscrew• If you're in Error you are one step away from the thumbscrews.• To open the lid, you simply loosen two thumbscrews. 