Originates from the End of the World flash movie, and was said by the Kangaroo before the world ended. Means literally "what the fuck, mate?" and is said in moments of confusion, surprise, or disgust. Is especially popular amoung slightly nerdy high school intellectuals who like flash movies like "Weebl and Bob" and "Homestar Runner."
Julie: Dude, we have a test in AP Chem today.
Bob: WTF, mate?
What the Kangaroo said in that one video where the world gets blown up by nukes.
Kangaroo: WTF, mate?