“Cornfielding” (or “banished to the cornfield”) is what happens when a post (or thread) is removed entirely by a mod. When it happens here, it’s nearly always because the poster is either a spammer or a sock.
The term comes from an episode of The Twilight Zone, called “It’s a Good Life,” in which a boy (played by Bill Mumy) could use his psychic power to cause things (or people) that displeased him to be sent away “to the cornfield,” never to be seen again.
