Green’s Dictionary of Slang

1. (US) to warn, to give a sign of recognition, to signal that there is no danger.

1924		‘The Bandit of Hell’s Bend’ Argosy All-Story Weekly Sept.–Oct. 🌐 She was pleadin’ with him, her eyes all teary and dim, / As I high-signed the barkeep for mine.
1925		G.M. Cohan Twenty Years on Broadway 46: And Josie started high-signing me again to quit ‘butting in’.
1935		A.J. Pollock Und. Speaks n.p.: High sign, to signal.
1939		R. Chandler ‘Trouble Is My Business’ in Spanish Blood (1946) 183: Hawkins took me to the elevator and highsigned me in.
1962		K. Orvis Damned and Destroyed 185: I noticed a house dick I know [...] I high-signed him to follow Moss’s pal upstairs.
1972		E. Folb Urban Black Argot 140: High Sign to display the colors, sign, etc. of one’s special group affiliation.

2. (US black) to show off, to upstage someone.

1972		E. Folb Urban Black Argot 140: High Sign to show off what one has, e.g. car, clothes, girlfriend, etc.
1980		E. Folb Runnin’ Down Some Lines 109: I’m always high signin’. Got to be clean and ready!
1985		T.R. Houser Central Sl. 27: high signin’ [...] ‘Hey look at Bobo with that new woman; he be high signin’.’.
1988		Ice-T ‘Soul on Ice’ 🎵 High-signin’ was his trademark, and he did it with class.
2001		(con. 1975–6) E. Little Steel Toes 63: The longer they gab and high-sign the longer we’re exposed.