A Soft-Drink popularized by pictures of Counter-Strike and World of Warcraft Junkies sitting at their desks full of trash and desperately needing a shave. Outside of the internet, it is mostly known for causing health problems (i.e. the Yellow 5 dye, which may cause problems in the genetaila.)
It should also be noted that nothing from Mountains makes Mountain Dew, and there is no Dew in fact. There may or may not be Pitch in Mountain Dew: Pitch Black. For those of you who don't know what Pitch is, google it.
Guy 1: Look at this dude! He's got what, 50 cans of Mountain Dew stacked up beside his desk?
Guy 1's GF: That's disgusting.