A: Don’t pay any attention to Bill. He says he know lots of celebrities, but I know it’s not true.

B: Absolutely. He’s full of hot air.

Note: This idiom means “full of nonsense” or “talking nonsense.” Used to describe exaggerations, empty talk, and obvious hyperbole, this slang term has its origins in the United States, and it appears to date to the late 1800s. As air heats, it expands. In the case of a hot air balloon, the air becomes lighter than the surrounding environment, allowing the balloon to float, so one could imagine someone being so full of hot air that he or she simply floats away. Or, the speech of someone like this tends to fill a space quickly, without offering much in the way of substance.