Another phrase for "smoking:... weed, marijuana, bud, etc." The word originates from Super Mario Bros. A "koopa" is a common creature, resembling a turtle with a green shell. An individual nug is referred to as a "koopa", thus "smoking the koopa."
Guy 1: Hey man, we should def pregame before we hit the club.
Guy 2: Hell ya hurry up and come thru with some drinks! Cause I'm already smoking the koopa.