Entry 1 sense Webster, 1913 Linchpin /(lĭnch"pĭn`)/ · Linch·pin · IPA /ˈlɪnt͡ʃˌpɪn/ 01 n. A pin used to prevent the wheel of a vehicle from sliding off the axletree. 1. A pin used to prevent the wheel of a vehicle from sliding off the axletree.