mid-14c., noumper, from Old French nonper "odd number, not even," in reference to a third person to arbitrate between two, from non "not" (see non-) + per "equal," from Latin par "equal" (see par (n.)). 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).
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Harper, D. (n.d.). Etymology of ump. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/ump