A stage whisper isn't a whisper at all. It's when you say something loud enough for everyone to hear, but soft enough for them to ignore you if they want to. This is particularly useful when you have something that's snide, potentially offensive or extremely clever to say. The term comes from the theater to describe an actor when his character needs to whisper on stage.

You can stage whipser in the Chatterbox by munging the "/msg" command ("/mgs", "/msd" and "/maf" are perennial favorites) so it looks like you're directing your message to a single person when you really want everyone to hear.