Da Eff?

1: A corruption of "da fuck"

2: A way of expressing that what was just said is confusing, stupid, or unexpected.

1: Dude: Hey man, I heard Joe jumped off a cliff!

Other Dude: Da eff?!

2: Dude: I think chickens are people too, man.

Other Dude: Da eff? What are you, retarded?