Longman Dictionary of Contemporary English homepage

Date: 1300-1400
Origin: linch 'pin fastening a wheel to an axle' (11-16 centuries) (from Old English lynis) + pin

linchpin

 noun
     
linch‧pin , lynchpin

the linchpin of something

the person or thing in a group, system etc that is most important, because everything depends on them
Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.

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