The act of wearing underwear, then a pair of gym shorts, then another pair of pants (could be gym shorts, jeans, sweat pants, cargo shorts or pants.) Each pair of pants/shorts sagging lower. Usually super low saggers do this. More shorts can be worn making it a "multi sag." (ex: boxers, gym shorts, gym shorts, sweats)
Carl: Woah did you see how low John was saggin at the party?
Trevor: Dude he was wearing boxers, gym shorts, and jeans that were fallin off his ass!
Carl: It was totally wack cool man!
Trevor: I'm gonna double sag next time, it's awesome!