A way of admitting a mistake, and apologizing for that mistake, without actually apologizing. The best definition I ever read of this, now paraphrased:
"I did something bad, and I recognize that I did something bad, but there is nothing that can be done for it now, and there is technically no reason to apologize for that error, so let's just assume that I won't do it again, get over it, and move on with our lives."
Ruder than apologizing, but with the same meaning: a flippant apology.
"You just spilled your beer on my term paper!!"
"Er... my bad."