biathlon (n.)

word-forming element meaning "two, having two, twice, double, doubly, twofold, once every two," etc., from Latin bi- "twice, double," from Old Latin dvi- (cognate with Sanskrit dvi-, Greek di-, dis-, Old English twi-, German zwei- "twice, double"), from PIE root *dwo- "two."

Nativized from 16c. Occasionally bin- before vowels; this form originated in French, not Latin, and might be partly based on or influenced by Latin bini "twofold" (see binary). In chemical terms, it denotes two parts or equivalents of the substance referred to. Cognate with twi- and di- (1).

"athletic contest of five separate events involving the same competitors and all taking place on the same day," 1650s, from Greek pentathlon "the contest of five exercises," from pente "five" (from PIE root *penkwe- "five") + athlon "prize, contest," a word of uncertain origin. Earlier in English in Latin form pentathlum (1706). The Greek version consisted of jumping, sprinting, discus and spear throwing, and wrestling. The modern version (1912) consists of horseback riding, fencing, shooting, swimming, and cross-country running. Related: Pentathlete.

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.