Source : Webster's Revised Unabridged Dictionary (1913)

Linchpin \Linch"pin`\ (l[i^]nch"p[i^]n`), n. [AS. lynis the
   axletree; akin to D. luns linchpin, OS. lunisa, LG. lunse, G.
   l["u]nse, OHG. lun peg, bolt.]
   A pin used to prevent the wheel of a vehicle from sliding off
   the axletree.

Source : WordNet®

linchpin
     n 1: a central cohesive source of support and stability; "faith
          is his anchor"; "the keystone of campaign reform was the
          ban on soft money"; "he is the linchpin of this firm"
          [syn: {anchor}, {mainstay}, {keystone}, {backbone}, {lynchpin}]
     2: pin inserted through an axletree to hold a wheel on [syn: {lynchpin}]