linchpin
A pin inserted through holes at the end of an axle or shaft, so as to secure a wheel or shaft-mounted device.
In ij camellis ferri vocatis lynspins emptis pro carectis iiijᵈ.
A central cohesive source of stability and security; a person or thing that is critical to a system or organisation.
figuratively
What is difficult to appreciate, however, is the discrepancy between his statement to the 'Manchester Guardian' correspondent and his known abhorance for party politics, which is the lynchpin of modern democracy.
Where it comes from
From Middle English lynspin, compound of lins (“axletree”) and pin, from Old English lynis (“lynchpin”), from Proto-West Germanic *lunis, from Proto-Germanic *lunaz – compare German Lünse and Dutch luns – from Proto-Indo-European. Possible further cognates are Welsh olwyn (“wheel”), Old Armenian ողն (ołn, “back; spine, backbone”) and Sanskrit आणि (āṇí, “lynchpin”). Figurative use attested from the mid-20th century.
Adapted from the Wiktionary entry linchpin (CC BY-SA 4.0).