Green’s Dictionary of Slang

six! excl.

(Can. Und./US prison) a shout of warning.

1978

R. Caron Go-Boy! 190: Buck started to keel over sideways just as some alert con yelled ‘Six!’.

In phrases

give six (v.) (also keep six) [SE give + six!]

(Can. prison) to keep a lookout.

1972		B. Rodgers Queens’ Vernacular 156: A comrade, poised as a lookout, is expected to give six [...] to sound a warning if authority approaches.
1978		R. Caron Go-Boy! 176: While I worked Ninny gave me ‘six’ by plastering her ear to the door and listening for activity out in the hallway.
1989		J. Morton Lowspeak 86: Keep six – prison term for keeping look-out.
2000		Other Side of the Wall: Prisoner’s Dict. July 🌐 Six: Keep watch, as in ‘Hey, Joe, keep six for me.’.
2006		Prison Slang Mommyblogger mydogharriet.blogspot.com 2 Mar. 🌐 You get that account back, and keep six on the guy.