modern composite Olympic event consisting of ten challenges, 1912, from deca- "ten" (from PIE root *dekm- "ten") + Greek athlon "contest, prize," which is of uncertain origin.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dasa, Avestan dasa, Armenian tasn, Greek deka, Latin decem (source of Spanish diez, French dix), Old Church Slavonic deseti, Lithuanian dešimt, Old Irish deich, Breton dek, Welsh deg, Albanian djetu, Old English ten, Old High German zehan, Gothic taihun "ten."
triathlon n.
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.