linchpin
n. US //ˈɫɪntʃˌpɪn// UK //lˈɪntʃpɪn// linch·pin
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.