When you leave someone up in the air, you leave the person waiting for a decision. You don’t tell him whether it’s a `yes’ or a `no’.
Examples: ”What do you mean you haven’t taken a decision about my promotion? How long do you plan to leave me up in the air like this?” * “The Vice Chancellor said that he hadn’t taken a decision about the demands. He left the students up in the air.” * ”Don’t leave me up in the air like this.”