The nasty fountain dispensed Mountain Dew that you order against your better judgement going in the drive thru of your local Taco Bell.
*Drive thru lady* "Would you like a drink with your 'Cheesy Double Beef Burrito?'"
*You* "And pay for that nasty fountain dew you're passing as drinkable liquid? I think not, I'll get a water."
(noun) Mountain Dew served from a restaurant fountain. Much fizzier, less caffeinated, and more watery than its bottled counterpart.
A: What do you wanna drink? They have Mountain Dew.
B: Nah. That's Fountain Dew. Apples and oranges, dude.