I find the expression puzzling; why not simply say "my mistake?"

I first heard "my bad" from my step-son when he was about 9. Seems he had been using that expression for quite a while (he is 15 now) and was even encouraged to use it. I gathered he had invented the phrase himself. I don't think he's the source of current wide-spread use of it, though. He doesn't get around _that_ much and neither rugby nor tennis is his bag.

(I love it. The spell-checker seems to think that "rugby" should be "rugged.")