A term commonly used when you are about to speak total bullshit for the next few moments. The statement after the phrase may be true, but the statement is irrelevant and doesn't apply to the situation
Saying this phrase to someone such as a teacher or significant other, will get you in trouble because by now they know this phrase. Saying it around your friends will cause them to laugh if you are seriously about to lie. They know it, I know it, your mom, dad, uncle, dog, and second cousin twice removed all know you're going to lie.
***You are in the principles officer after sleeping late and you recall yesterday you had a flat tire***
Principle: Little Jimmy, why were you late again this morning.
Little Jimmy: Well... uh.. you see what had happened was
... was that I had a flat tire and I could not make it to school on time because I cannot drive with a flat tire.