A floatie is an unfinished beverage, containing perhaps an inch of liquid therein. Some people have a habit of leaving floaties - claiming that the last gulp of pop is 90% saliva, or that the last bit of beer is always flat - although their excuses are often untrue.
Floaties often get tipped over, and make a big mess. The least dangerous type of floatie is water in a transparent pop bottle, as it's obviously full of something, so shouldn't be bumped, and doesn't stain if it does spill. The messiest type of floatie is half a beer, with plenty of cigarette butts therein, in an aluminum can. It just waits, waits for its chance to cover my possessions in its gooeyness.