It’s a sign that’s used in a way other than the intended one. So are the others. What is a sign? It’s something that signifies a message. The thing that’s doing the signifying, call it the signifier, that’s stable. The message, on the other hand, the thing being signified (and we’ll call that the signified), that’s up for grabs. The signifier, in other words, while being fairly stable itself, doesn’t have to be used in the planned way. Its meaning can be deflected from the expected meaning.