I suppose it just refers to the hot air coming out of your mouth. That is, it might as well just be hot air as the words have no real meaning.

Actually, the earliest record of this idiom was most likely written by the author Mark Twain. In the book titled Gilded Age (written in 1873), he wrote "The most airy schemes inflated the hot air of the Capital." This was in reference to the lies politicians often tell. And it just means someone is either pontificating or saying a lot of words that are simply nonsense. Sounds about right when you think of the majority of politicians in the world. =P

I believe the expression may have originated around the time the Montgolfier brothers invented the "hot air" balloon. This was an extremely dramatic device for lifting people into the air but, despite it's dramatic appearance is was only a cloth bag. There was no substance, in effect, just "hot air."