THUMBSCREW
thumbscrew
(noun) screw designed to be turned with the thumb and fingers
thumbscrew
(noun) instrument of torture that crushes the thumb
Source: WordNet® 3.1
Etymology
Noun
thumbscrew (plural thumbscrews)
A screw that can be turned with the thumb and fingers.
An instrument of torture used to crush the fingers.
(figuratively) A weakness that can be taken advantage of.
Synonyms
• (torture instrument): pilliwinks
Source: Wiktionary
Thumb"screw`, n.
1. A screw having a flat-sided or knurled head, so that it may be
turned by the thumb and forefinger.
2. An old instrument of torture for compressing the thumb by a screw;
a thumbkin.
Source: Webster’s Unabridged Dictionary 1913 Edition