Green’s Dictionary of Slang

high sign n.

also hi-sign

1. (orig. US) a warning, a recognition signal, a secret sign, esp. when denoting one’s membership of a group; a signal that the ‘coast is clear’.

Switchmen’s Jrnl 582: Rees giving a high sign to stop, and if you see a short, dark man running in and out amongst the cars don't take him for an Indian; it is only Bro. C. F. Oakins .
Railroad Trainman 13 807/1: The train [...] is manned with reliable, sober and loyal men, and I proudly give the ‘high sign,’ feeling certain that no competing or opposition line can offer the safety, comfort and protection afforded by yours.
[US]‘O Henry’ ‘Whistling Dick’s Christmas Stocking’ in Roads of Destiny (1914) 231: Dat man [...] is a dead ringer for Boston Harry. I’ll try him wit’ de high sign.
[US]W. Irwin Confessions of a Con Man 30: He gave me the high sign of the profession to show he was all right.
[US]J. Lait ‘Charlie the Wolf’ in Beef, Iron and Wine (1917) 69–70: The warden and his fam’ly is sittin’ over on a bench, and they gives each team the high-sign o’ success and good wishes.
[UK](con. 1917–18) J.M. Saunders Wings (1928) 202: I’ll give you the high sign when it’s time to get up.
[US]C.L. Edholm ‘Gorilla Girl’ in Gun Molls Oct. [Internet] ‘All set,’ he remarked. ‘Give Cora the high sign.’.
C.B. Yorke ‘Snowbound’ in Gangster Stories Oct. n.p.: Francis gave me the high sign.
[US]D. Maurer Big Con 242: [He could] give his lawyer the high sign.
[US]D. Dressler Parole Chief 226: He was giving me the office – that is, an unspoken high sign that means, ‘Nix, brother.’.
[UK]R. Cook Crust on its Uppers 71: I give her the hi-sign.
[US]C. Himes Cotton Comes to Harlem (1967) 186: Paul had just ridden by giving the high sign that Iris was still put.
[UK]N. Smith Gumshoe (1998) 23: The boss of the place stuck a wad of notes in his hand and gave him the high sign.
[US]R. Campbell Alice in La-La Land (1999) 36: The waitress hadn’t even come to take their orders yet, because nobody had given her the high sign they were ready.
W. Warga Singapore Transfer 135: I saw Davidson give him the high sign from across the room.
G.W. Haslam Haslam’s Valley 71: ‘Sure ya can,’ snapped Earl, and I give him the high sign.

2. a sign to leave, a rejection; as give someone the high sign.

[US]W.R. Burnett Little Caesar (1932) 12: Give them the high sign, and they’ll beat it.
[US]J. Tully Shadows of Men 199: I went on the hop for her. Then, when I couldn’t ride no more, she wants to give me the high sign an’ run away with a travelin’ salesman.