Why and when did the word hop become used in reference to a dance?
Historyspaces claims Milwaukee and beer.
Others mention Lindy Hop (Charles Lindberg, 1920s) and sock hops (1950s).
The earliest reference I’ve seen is from etymonline with this intriguing entry:
This word [hop] has always been used here as in England as a familiar term for dance; but of late years it has been employed among us in a technical sense, to denote a dance where there is less display and ceremony than at regular balls. [Bartlett, "Dictionary of Americanisms," 1848]