Usually occurring at a concert, in an atmosphere with lot's of people smoking (most likely weed) and exhaling mass amounts of smoke into the crowd. You then inhale a mass amount of this second-hand smoke, and end up high.
Person 1- "Man, you are SO high."
Person 2- "Nah man, there was just SO much smoke at that concert, I probably got second-hand high."
Person 1- "Makes sense dude, makes sense."