Tennessee bottlers Barney and Ally Hartman developed Mountain Dew as a mixer in the 1940s. Soft drinks were sold regionally in the 1930s, and the Hartmans had difficulty in Knoxville obtaining their preferred soda to mix with liquor, preferably whiskey, so the two developed their own. Originally a 19th-century slang term for whiskey, especially Highland Scotch whiskey, the Mountain Dew name was trademarked for the soft drink in 1948.

Source: Mountain Dew