1) To smoke up in a small room so that the room becomes filled with thick smoke, thus enhancing your high. To sub out is the same as to hotbox, fishbowl, or clambake.
2) To remove yourself from a sports game so that another player can take your place.
Coach K: "Are you all right, J.J.? You look a little out of it today. Maybe you should sub out."
J.J. Redick: "Coach, I already subbed out before the game, and I'm not just out of it, I am royally FUCKED UP!"