The phrase was said by the infamous LIL MAMA on the show "america's next best dance crew" on MTV, obviously. One of the groups(super crew) to be exact really shocked her by their sweet dancin' skillz, evidently she didn't know what to say other then .. "YA'LL TURNT IT UP"
Basically, it's when someone does something so unremarkably amazing that there's nothing left to say other then them turntin it up. You're either a natural turnt it or you're not. Sadly some people are natural turn it down's.. they can redeem themselves but it'll take a lot of practice and energy.
Lil Mama: I don't know what to say, ya'll turnt it up!