Definition of "thin air" in English

Noun

1. An unknown location; apparent disappearance or nonexistence.

   • Where did you think the exhaust would go? Into thin air?

2. Used other than figuratively or idiomatically: see thin,‎ air,‎ (especially, low-density air as a result of high altitude).

   • You'll need to stay mindful of taking breaks, drinking water, and avoiding overexertion up here, because you're dealing with thin air at this altitude.