Why is the ninth month called September?
c.1400, noumper, from Old French nonper "odd number, not even," in reference to a third person to arbitrate between two, from non "not" + per "equal," from Latin par. Initial -n- lost by mid-15c. due to faulty separation of a noumpere, heard as an oumpere. Originally legal, the gaming sense first recorded 1714 (in wrestling).
c.1600, from umpire (n.). Related: Umpired; umpiring.