I found this…..
"A highball is a quickly poured drink served in a highball glass with liquor and water or soda. The phrase ‘highball’ comes from the practice of placing a ball on a tall pole in a railway station to signal a train that it was running behind schedule. The reason why the name was attached to the drinks is because the highball is a drink quickly made, with at most one liquor, one mixer, and only a single garnish………."
The article in its entirety can be found here: http://www.drinkstreet.com/category.cgi?category=10
Hope this helps,