Challenger

wiktionary

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From Middle English chalenger; equivalent to challenge +‎ -er.

etymonline

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challenger (n.)

late 13c., "a claimant;" mid-14c., "one who makes false charges;" mid-15c., "one who disputes something, objector," from Anglo-French chalengeour (Old French chalongeor "slanderer, petitioner, plaintiff"), agent noun from challenge (v.). Specific sense of "one who calls out another in a contest" is from 1510s.