Green’s Dictionary of Slang

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’.

1892		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 .
1896		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.
1899		‘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.
1909		W. Irwin Confessions of a Con Man 30: He gave me the high sign of the profession to show he was all right.
1916		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.
1927		(con. 1917–18) J.M. Saunders Wings (1928) 202: I’ll give you the high sign when it’s time to get up.
1930		C.L. Edholm ‘Gorilla Girl’ in Gun Molls Oct. 🌐 ‘All set,’ he remarked. ‘Give Cora the high sign.’.
1931		C.B. Yorke ‘Snowbound’ in Gangster Stories Oct. n.p.: Francis gave me the high sign.
1940		D. Maurer Big Con 242: [He could] give his lawyer the high sign.
1951		D. Dressler Parole Chief 226: He was giving me the office – that is, an unspoken high sign that means, ‘Nix, brother.’.
1955		B. Appel Tough Guy [ebook] [He] gave her the high high-sign, slapping his left hand down on the bicep of his right arm as he jerked his right forearm up.
1962		R. Cook Crust on its Uppers 71: I give her the hi-sign.
1965		C. Himes Cotton Comes to Harlem (1967) 186: Paul had just ridden by giving the high sign that Iris was still put.
1971		N. Smith Gumshoe (1998) 23: The boss of the place stuck a wad of notes in his hand and gave him the high sign.
1987		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.
1991		W. Warga Singapore Transfer 135: I saw Davidson give him the high sign from across the room.
2005		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.

1929		W.R. Burnett Little Caesar (1932) 12: Give them the high sign, and they’ll beat it.
1930		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.