A primitive home security technique. In some old homes with multiple stories, it was commmon to have a door on the highest floor that says "Treasury: Do Not Enter" or something equally enticing. This door, however, would not lead to a treasury, but to nothing but thin air. The would-be criminal would then, of course, be fucked.
"Boy, honey, good thing I put in that Irish elevator last summer!" Bob exclaimed as he peeled burglar bits off the lawn.