[Coda]

Actually I think (though I'm not certain) that it's "nani yatten da yo" which would mean "what are you doing?!", or possibly "atten da", which would make it "what is it?!" Both make sense and I've never seen it written out to know which it is.

Breaking it down: First, "nani" is "what". After that, you have "yatten da". This is "yaru" ("to do", rude) in -te form (making it "doing"), plus -n da for emphasis. And then "yo" for even more emphasis.

If it's "atten da" instead of "yatten da", then the verb is based on "aru" ("to be", casual) under the same conjugation.

Edit: *does a bit of research* It's "yatten da".