Locknut

A locknut, also known as a lock nut, locking nut, prevailing torque nut, stiff nut or elastic stop nut, is a nut that resists loosening under vibrations and torque. Elastic stop nuts and prevailing torque nuts are of the particular type where some portion of the nut deforms elastically to provide a locking action. The first type used fiber instead...
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locknut

safety nut noun supplementary nut that is screwed down on a primary nut to prevent it from loosening
Found on https://www.encyclo.co.uk/local/20974

Locknut

a nut designed not to loosen due to vibration.
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locknut

[n] - supplementary nut that is screwed down on a primary nut to prevent it from loosening
Found on http://www.webdictionary.co.uk/definition.php?query=locknut
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