the.com/eventing

means An equestrian discipline where a single horse-and-rider pair competes across three phases — dressage, cross-country, and show jumping — over one event, testing obedience, speed, and precision in turn.

from The word is simply 'event' put to work as a gerund: a competition made of events plural, bundled into one. 'Event' itself traces back through Old French to the Latin 'eventus' — an outcome or occurrence, from 'evenire,' to come out or happen. The riding sport borrowed the ordinary word for 'a scheduled happening' and let it harden into a proper noun for this specific triathlon-on-horseback, likely because early competitions were literally described as combining several 'events' into one test of the all-round horse.