The most annoying comeback phrase to express rejection known to man. Usually used by douche bags and immature people who cannot/ don't know any other way of expressing themselves therefore use this "trendy" phrase. Emphasis is usually put on NO while umm is spoke at a normal tone.
Person A: Hey lets go see Titanic.
Person B: Umm..NO how about we see Twilight instead.
Person A: Wow...That was kinda harsh..