Green’s Dictionary of Slang

(drugs) a dark, oily substance that remains in a pipe after crack or freebase n. (1) cocaine has been smoked.

1989		(con. 1982–6) T. Williams Cocaine Kids (1990) 6: Some used the residue (called ‘due’ or ‘con-con’) in the form of an oil to saturate joints.
1992		T. Williams Crackhouse 74: The kids in my old block used to soak Bambu paper with the con-con, roll a cigarette and smoke it, and call that bazuca. Now they do something quite different. They mix dope [heroin] and crack in the pipe.