A climbing game similar to HORSE in basketball. Two or more climbers will alternately climb up a wall/shelf/boulder one or two holds at a time in attempt to make their move too difficult for their opponent to complete. Each time a mistake is made or a player falls a letter is added. Whoever does not have ADD ON spelled by the time everyone else does is the winner.
This game is common among bouldering cllimbers.
Climber1: "Hey dude wanna play add on?"
Climber2: "Yeah. I hope you can set on a backstep to a pinch hold, bitch!"