Mencken mentions "highball" as an alcoholic drink. I suspect it was borrowed from railroad word.

"The Americans, introduced to the same drink, at once gave it the far more original name of high-ball."

The highball was the signal the conductor at rear of train gave by waving hand or a lantern in an arc high overhead, meaning it was OK to start the train towards next stop.

Talking about cocktails, Dr Bill, what about the Sidecar, is that also railway related?

My guess would be motorcycle. I don't get the figure of speech involved though.