Here's the breakdown:
* "Upset" as a term in horse racing emerged in the early 20th century, referring to a horse that was considered a long shot (meaning it had low odds of winning) but surprisingly won the race. This term came into prominence after the famous racehorse "Man o' War" was unexpectedly defeated by a horse named "Upset" in 1919.
* "Pull an upset" evolved from this racing context. It became a common way to describe any situation where an underdog or less-favored entity unexpectedly triumphs over a stronger or more likely contender.
While the exact origin of the phrase isn't definitively documented, its association with the "Upset" racehorse and the broader world of horse racing is strongly established.