The phrase "upset loss" refers to a situation where a favored or expected team or individual loses to an underdog or unexpectedly loses a competition or game. Full definition
If it weren't for a 1 - 0 upset loss in the same title game to Yucaipa in 2015, we'd be writing about Mission Viejo as being State Team of the Year three straight seasons. (sportstarsmag.com)
But the Trojans also have one of the biggest upset losses of all time (Stanford). (sbnation.com)
They host a Saints team that is coming off an ugly upset loss at home against the Lions. (sportsinsights.com)