Origin[edit]
The meaning of the word "upset" has long included "an overthrowing or overturn of ideas, plans, etc." (see OED definition 6b), from which the sports definition almost surely derived. "Upset" also once referred to "a curved part of a bridle-bit, fitting over the tongue of the horse", (now the port of a curb bit), but even though the modern sports meaning of "upset" was first used far more for horse races than for any other competition, there is no evidence of a connection.
In 2002, George Thompson, a lexicographic researcher, used the full-text online search capabilities of The New York Times databases to trace the usage of the verb to upset and the noun upset. The latter was seen in usage as early as 1877.[1]
Thompson's research debunked one popular theory of the term's origin, namely that it was first used after the surprising defeat of the horse Man o' War by the racehorse Upset in 1919 (the loss was the only one in Man o' War's career).
The term pre-dates that 1919 Thoroughbred race by at least several decades. In its sports coverage immediately following Upset's victory, the Washington Post wrote, "One might make all sorts of puns about it being an upset."
The name of the horse "Upset" came from the "trouble" or "distress" meaning of the word (as shown by the parallelism of the name of Upset's stablemate, Regret).