Devil-may-care - Meaning recklessly free of spirit.
I've always thought this phrase, as in "He has a devil-may-care attitude" meant something along the lines of - "The devil may worry about what I'm doing, but I sure as hell don't." In other words, I'm so reckless even the devil is concerned. But, I figured if I ever investigated it I'd find some mundane, boring origin.
I was wrong. The phrase is literally a shortening of "The devil may care, but I do not."