mid-15c., "engage in rivalry, compete," from Old French contendre and directly from Latin contendere "to stretch out; to shoot, hurl, throw; strive after mentally; measure or try one's strength with, fight, vie with," from assimilated form of com-, here perhaps an intensive prefix (see com-), + tendere "to stretch" (from PIE root *ten- "to stretch"). From 1540s as "to assert, affirm, maintain." Related: Contended; contending.
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Harper, D. (n.d.). Etymology of contender. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/contender