The same as facepalm, which is used to show frusteration at a certain topic/post. Taken from The Dark Knight scene where the Joker slams a man's head in to a pencil after saying he would show them a magic trick and make the pencil "disappear."
Fool: Hey guys, I never watched Friends. Is it a good show?
Cool guy: .../facepencil
To ram someone's head into a pencil. Often said when you are frustrated or annoyed. A variation of the word facepalm. Stems from a very famous scene in the movie The Dark Knight, when the Joker rams a bouncer's head into pencil.