Middle English lynspin, compound of lins 'axletree' and pin, from Old English lynis 'linchpin', from Proto-Germanic *luniso (compare German Lünse), from Proto-Indo-European (compare Welsh olwyn 'wheel', Armenian ołn 'shoulder', Sanskrit āṇís). Figurative use attested from the mid-20th century.
Alternative forms 
linchpin (plural linchpins)
- a pin inserted through holes at the end of an axle, so as to secure a wheel
- (figuratively) a central cohesive source of stability and security; a person or thing that is critical to a system or organisation.