linchpin

There was the good old custom of taking the linch-pins out of the farmers' and bagmens' gigs at the fairs, and a cowardly blackguard custom it was. That custom, described by British writer Thomas Hughes in his 1857 novel Tom Brown's School Days, was "blackguard" indeed. The linchpin in question held the wheel on the carriage and removing it made it likely that the wheel would come off as the vehicle moved. Such a pin was called a "lynis" in Old English; Middle English speakers added "pin" to form "lynspin." Modern English speakers modified it to "linchpin" and, in the mid-20th century, began using the term figuratively for anything as critical to a complex situation as a linchpin is to a wagon.