1970, from tri- "three" + Greek athlon "contest;" formed on model of decathlon, biathlon, etc. Originally of various combinations of events; one of the earliest so called combined clay-pigeon shooting, fly-fishing, and horse-jumping; another was cross-country skiing, target shooting, and a giant slalom run; and a third connected to the U.S. Army involved shooting, swimming, and running. Applied to the combination of a long swim, a bicycle-race, and a marathon by 1981.
early 15c., from Latin athleta "a wrestler, athlete, combatant in public games," from Greek athletes "prizefighter, contestant in the games," agent noun from athlein "to contest for a prize," related to athlos "a contest" and athlon "a prize," which is of unknown origin.
Until mid-18c. usually in Latin form. In this sense, Old English had plegmann "play-man." Meaning "Anyone trained in exercises of agility and strength" is from 1827. Athlete's foot first recorded 1928, for an ailment that has been around much longer.