Big deal, important occurance. Usually used sarcastically or to blow something off.
Your friend tells you someone is talking smack about you behind your back, and you don't want her to know it bothers you, but irritated you reply,"Oh, big whoop!"
A friend accidently spills soda on your floor and is all bent out of shape and apologetic about it. You wipe it up and tell her, "There, all fixed, no big whoop."