Anything that’s up in the air is uncertain or iffy. Are your birthday plans up in the air? That means you haven’t decided yet what you’ll do to celebrate.
Use the phrase up in the air to talk about something that’s undecided or unresolved. If your teacher hasn’t decided whether or not to give a final exam, it’s up in the air. And if you have a bad cold today, the possibility of going on a picnic this weekend is still up in the air. We’ve been using up in the air this way since the 1750s.