Okay, so what's the Western Wall in Jerusalem for?

wall(8) (previously invoked from /bin) is a Unix command to "write all", and wall used to emit a humorous error message, which IIRC was output when you provided no text to send as a message:

  But what do you want to do with the wall?

And I believe this was, in turn, a reference to adventure games like Infocom's, where the parser may detect that you typed a recognized noun without a verb in front, like "hit wall" or "push wall" and give exactly the same message as a prompt to include a verb.