JavaScript Teaser: Awful Functions

Here is our JavaScript Teaser for May 14, 2015. Can you solve it?

What does this function return and why?

function awful() {
  function terrible(x, y, z) {
    return x;
  }
  terrible[0] = "this";
  terrible[1] = "is";
  terrible[2] = "pretty";
  terrible[3] = "terrible!";
  terrible.length = 4;
  return [].map.call(terrible, terrible).join(" ");
}

Please post the answer on JSFiddle or similar service, and then post a link to your answer here. You can also post comments below or email Rangle.io's CTO, Yuri@rangle.io, if you wish to share further ideas.