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.
athlete n.
early 15c. (Chauliac), "competitor in athletic games and contests," from Latin athleta "a wrestler, athlete, combatant in public games," from Greek athlētēs "prizefighter, contestant in the games," agent noun from athlein "to contest for a prize," which from athlos "a contest," especially for a prize (its neuter form, athlon, meant "the prize of a contest"), a word of unknown origin.
Until mid-18c. it was usually in English in Latin form. Old English had plegmann "play-man." The meaning "one trained in exercises of agility and strength" is by 1827. Athlete's foot is recorded by 1928, for an ailment that has been around much longer.