You say this when you ask a Question and someone Answers with "I Don't Know". Then you say "Well, If You Did Know What Would You Say?" Then you have a great feeling in your stomach and the other person feels like a stupido. Haha lib.
Tom: Yo, What Time Is It?
Jerry: I Don't Know
Tom: Well, If You Did Know What Would You Say?
Tom: Well, You Should Know!
Jerry: Ok, I'll Go Check