Thumb nut

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a nut for a screw, having wings to grasp between the thumb and fingers in turning it; also, a nut with a knurled rim for the same perpose.

See also: Thumb

Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.
References in periodicals archive ?
No problem, as your reading was locked in by tightening the knurled thumb nut.
* With you fingers, loosen the thumb nut counter-clockwise.
The handle is loosened by a thumb nut on the bottom." Stanley recalls a cautionary note.