1. An indication of confusion, usually spoken with a quizzical tone: “Wall??”
2. A request for further explication. Compare octal forty.
3. [Unix, from ‘write all’] v. To send a message to everyone currently logged in, esp. with the wall(8) utility.
It is said that sense 1 came from the idiom ‘like talking to a blank wall’. It was originally used in situations where, after you had carefully answered a question, the questioner stared at you blankly, clearly having understood nothing that was explained. You would then throw out a “Hello, wall?” to elicit some sort of response from the questioner. Later, confused questioners began voicing “Wall?” themselves.