Various explanations are offered for this term of the American drinking vocabulary. While increasingly a period expression, most interested in drinks history know the highball is a tall glass of whiskey, diluted with ice and water or seltzer, and later pop of some kind (although whiskey-and-coke is not really a highball, IMO).
Some say the highball term derives from a mechanical signal in the 1800s on the American railways. If a ball was raised high on the signal post, the train could drive through at speed. So, the idea of getting somewhere fast on the rails was transferred to drinking: one obtained a quick buzz from drinking a whiskey highball. There is a French term, “Rapide”, for an alcohol concoction that may have a similar, or possibly still unrelated, rationale.
This explanation always struck me as contrived. Moreover, a long drink doesn’t intoxicate quickly, it is meant to be sipped, and imparts its effects gently. It is rather a short neat drink that would intoxicate faster, but that is not a highball.
I believe I have an original explanation, at least, I can’t recall having read it elsewhere (citations or comments always welcome).
“Ball of malt” is Irish usage for a glass of whiskey, meaning a small measure neat. Many sources attest to this, no controversy attaches. It might derive from “boll of malt”, a measure of barley in former times, and been transferred to the drink the malt makes.
So, a highball is a tall glass of whiskey, watered that is, nothing more.
True, the Irish sometimes add water to whiskey, but traditionally not a lot, as in Scotland, it is still a short drink. 50/50 was traditional, no ice, Especially in earlier visits to the U.K. I saw whisky drunk that way many times.
Irish customs influenced American distilling practice, via the Scots-Irish emigrants who had a large role in developing rye whiskey (in Pennsylvania), the antecedent of bourbon whiskey. Both these American forms use a mixed grain mash, as does still Irish single pot still whiskey, the traditional type vs. the more recent Irish single malt.
Later, we infer the sizable emigration from what is now the Republic of Ireland added more Irish whiskey knowledge and lore to American drinking folkways. I think ball of malt likely was shortened to ball, meaning a short drink of whiskey. When people poured whiskey into tall glasses in America, ergo the highball. No need for belaboured explanations relating to old railway technology.
It would be satisfying to find a citation for lowball to mean a short measure of whiskey in the U.S., that would tend to clinch it. I’ll look.