locknut

A usually thin nut screwed down on another nut to prevent it from loosening.

A self-locking nut.

A supplementary nut screwed down upon another to prevent it from shaking loose; a jam-nut, check-nut, or pinch-nut.

a supplementary nut that is screwed down on a primary nut to prevent it from loosening; a check nut.

A second nut, screwed down onto another in order to prevent it slipping

supplementary nut that is screwed down on a primary nut to prevent it from loosening