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.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.