WWWJDIC: [天地無用]{てんちむよう} (exp) (yoji) do not turn upside down
I see this on some cardboard boxes in the context of shipping and handling. Why is this expression used to indicate "this side up", or where does it come from?
I can understand that 天 and 地 refer to up and down, but how does 無用 (useless, futile, prohibited) play a role in this phrase?