1
general::
Not settled, uncertain, as in The proposal to build a golf course next to the airport is still up in the air. This metaphoric expression likens something floating in the air to an unsettled matter. Put as in the air from the mid-1700s, it acquired up in the first half of the 1900s.
American Heritage Idioms
2
general::
Phrase(s): up in the air (about someone or something)
Fig. undecided about someone or something; uncertain about someone or something. • I don’t know what Sally plans to do. Things were sort of up in the air the last time we talked. • Let’s leave this question up in the air until next week.
McGrawhill's American Idioms And Phrasal Verbs