1. cease to be visible in a sudden and mysterious way
it seemed to just vanish into thin air!
This is often used in a sarcastic manner, considering that nothing can just disappear magically. If you say that something 'vanished' or 'disappeared' into thin air, you mean that you have no idea where it went. People might look everywhere to find something - or even someone - and find no trace of it (or them). That is when they might say, "It just vanished into thin air!"