Meaning of thin air | Babel Free

Definitions

1. An unknown location; (by extension) apparent disappearance or nonexistence.
   figuratively, humorous, idiomatic, uncountable, usually
2. Used other than figuratively or idiomatically: see thin, air, (especially, low-density air as a result of high altitude).
   uncountable

Examples

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

“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.”

CEFR level