Roach Joint means in slang: One joint made up of the roaches from other joints. This makes for great smokage. A “roach joint” is actually a “grandfather-roach”, which refers to a piece of marijuana that somehow managed to survive two joints. A “grandfather joint”, in turn, is a joint that has been rolled and contains grandfather roaches. It’s a great way to have fun with friends. (in Slang Dictionary, added by Damián López)