The name "highball" probably refers to the practice of serving drinks in tall 12 ounce glasses.
Another theory is that it's named for the dining cars of trains powered by steam locomotives; when the engine would get up to speed and the ball that showed boiler pressure was at its high level, known as "highballing".
Seeing as it's nothing but spirits – whiskey originally, then whatever – ice, and soda water or ginger ale (an early innovation), you'd think it wouldn't take much inventing, but before then no red-blooded American drinker would have conceived of introducing water, of all things, into good red whiskey, tiny bubbles or no. So they say, anyway.
Nowadays orders as varied as "Dark and Stormy's" and "Moscow Mules" are classified as highballs.
Whatever your preference, enjoy a highball on 2-4-1 Wooden Nickel Wednesday. Have the second one today or any other day.