Usually said when someone asks you a question that you really don't want to answer, but they won't leave you alone until you answer it. Usually you should say this when your little brother asks you a stupid question and it will cause him to leave you alone and ask your parents.
Little Bro- " Why do girls have cooties???
Me * precedes to browse twitter and facebook*
Little Bro- " I said, Why do girls have cooties!!!!"
Me " How would I know you spoiled piece of shit!!! Get the fuck outta here before I back hand your remedial ass!!"