Green’s Dictionary of Slang

(drugs) the cocaine oil that remains in a pipe after freebasing.

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 130: ‘I traveled all over the world’ [...] he mutters, pushing a stick through one end of the pipe to move the ‘due’ (residue) on the screens back to the other end before lighting it again.
2001		ONDCP Street Terms 8: Due — Residue of oils trapped in a pipe after smoking base.