When one lies in a stiff horizontal position (sometimes naked) on random objects or in random places. Eg) Walls, hedges, fire hydrants, cars, fences, lights, signs, trolleys etc. Usually accompanied by someone who takes a photo and posts it on the internet.
Catherine: dude can we go planking next week end in the city?
Emma: yeah that's MADD! Let's plank on the mcdonalds drive though sign!