Some gay shit people do in order for other people to laugh at them. It's thought that it was originated at some gay frat guy's house then spread to all the fucking faggots out there.
To lay completely rigid, face down on any surface. Your arms must be placed by your side, held straight and fingers pointed. Must also have a expressionless face.
Other known as "The Lying Down Game" (UK)
To lie face down with an emotionless face, arms to the side, in a rigid manner. Can be done by anyone on anything to make an impressive picture. It is proper to have a picture taken of oneself planking and post it on Facebook. Originated by British Rugby player David "Wolfman" Williams
When a person or persons lie flat and rigid face down with both palms pointed down, usually across two surfaces not meant to be lied on. Originating in Australia
Jeff: check out my mad planking skills
Frank: dude you just planked across two buses, this is going onfacebook