Umpire began as noumpere, from the Old French nonper: not equal, without a peer. It meant the odd one, the third person called in when two arbitrators, one chosen by each side, could not agree. By the late 1300s “a noumpere” was being heard as “an oumpere”, the same slip that turned a napron into an apron, and the n never came back. What stayed is the idea. Everyone in a dispute has a pair: the other side, their own arbitrator, someone who matches them. The umpire is the one person present who matches nobody, and that is the whole qualification. To judge between equals, you have to be the only thing in the room that isn’t one of them.