When you ask for a cup at a fast food restaurant so you can get water at the soda fountain, but later get soda instead. For paranoid types, this usually works best with clear sodas, like Mountain Dew or Ginger Ale.
Dude, why is your water bubbling?

That's not water, man....it's secret soda!
by Carbonation Jane June 28, 2012
Get the secret soda mug.