When did the word upset come to mean an unexpected victory?

Answer 1

When a horse named "upset" beat an undefeated horse named "Man of War"

That's a common misconception, but the use was in widespread use prior to that race in 1919 (examples below)...those who believe that the horse Upset caused the use of the term would have to explain why the horse was named that (was the owner "upset" about something and named the horse that? Doubtful...instead, it's far more likely it was named Upset because the term was already in common use):

<1877 "The programme for to-day at Monmouth Park indicates a victory for the favorite in each of the four events, but racing is so uncertain that there may be a startling UPSET."-'New York Times,'17 July, page 8> [[from the research of George Thompson, courtesy of Dave Wilton]]

<1887 "It [[the horse race]] was such an UPSET as Mr.Kahn, the owner of Saxony, had not deemed possible, and he naturally felt very much annoyed by it."-'New York Times,' 24 June, page 2>

<1888 "The Suburban Renewal brought only three horses to the post, but it furnished an UPSET at that. With only Le Logos and Ordway against him, Dry Monopole was an odds on favorite . . . Dry Monopole fell back and lost the [[first ]] place to Le Logos."-'New York Times,' 17 June, page 16>

<1890 "With Tipstaff the hottest sort of favorite for the opening race, a three-quarter-mile dash the sport began with an UPSET, for Madstone, a 3 to 1 shot, landed the prize in good shape, beating the favorite home by a length."-'New York Times,' 30 May, page 3>

<1895 "Stevens was tiring Wren out rapidly, and had the [[tennis]] contest, which was postponed from Wednesday, not been shortened to two in three, the Jerseyman would have furnished an UPSET." 'Chicago Daily Tribune,' 11 August, page 4>

<1902 ". . . but nothing is more uncertain than form in football, and an UPSET may occur in any one of the four contests."-'New York Times,' 24 November, page 11>

<1903 "Horton was bred near Vàllejo by the late 'By Holly,' and is seven years of age. There was an UPSET [[horse racing]] in the last race, when Hermencia outlasted Diderot and beat him."-'New York Times,' 8 February, page 11>

<1915 "Yeager scored an UPSET [[in tennis]] by eliminating Buynitsky, Hamilton Park, ranked as the favorite before the semifinals."-'Chicago Daily Tribune,' 6 September, page 12>

<1916 "Everything points to and Illinois [[football]] victory. but there is more than a possibility of an UPSET."-'Chicago Daily Tribune,' 18 November, page 9>