Green’s Dictionary of Slang

highball n.2

[the small hanging ball used as a signal]

(US) a signal, orig. used by railroads, meaning ‘proceed’.

[US]Chicago Record 1 Mar. 6/1: ‘Milk trains’ [...] have ‘rights’ over the rails and get nothing but ‘high balls’ [DA].
[US]Fr. Klaeber ‘A Word-List From Minnesota’ in DN IV i 11: high-ball, n. Signal, or order (to go). (Railroad term.) ‘The conductor gave the engineer the high-ball to go ahead.’.
[US]‘Digit’ Confessions of a Twentieth Century Hobo 70: The conductor waved his lamp—two short, sharp whistles, the highball, and we started to pick up speed rapidly.
[US](con. 1890) G. Milburn ‘A Convention Song’ in Hobo’s Hornbook 27: Toot! Toot! there goes a highball now; / The rattler’s under way.
[US]F.H. Hubbard Railroad Avenue 1: ‘Highball’ is the go-ahead or come-on signal.
[US]L. Brown Iron City 33: That’s called highball. Railroad signal, means let’s go!
[US]Ragen & Finston World’s Toughest Prison 803: high ball – A ‘proceed’ signal.
[US]Current Sl. V:1.
[US](con. 1930s–60s) H. Huncke Guilty of Everything (1998) 238: We waited ’til they linked up the various cars and gave the highball, and then we caught it on the fly.

In phrases

give someone the highball (v.) [? railroad jargon highball, a fast train; thus the individual who ends the affair is fig. ‘taking a fast train’ out]

(US) to reject, to brush off, esp. to end a relationship or love affair.

[US] in DARE.
Syl Francis ‘Little Boy Lost’ Pt 10 on Junkheap 🌐 ‘Go on, give him the highball,’ Tommy broke in. ‘We can’t transport a screaming kid across the city, and we’ve got to lay low all day! You *know* what they’ll do to us if we’re found with a kid!’.