no fair using double posts

What's with this "no fair" business? My children sometimes use the phrase, which I have assumed - rightly or wrongly - that they have picked up from (US?) television programs.

To me, "it's no fair" would mean the same as "it's no picnic" (i.e. hard work, not much fun), but people use it for "it's unjust" where I would use "it's not fair".