It is an exclamation used to describe an act that is astounding beyond words. So you say- WHAT THE FLAGNOG?!?!? (It has also been used as a varition of WTH or WTF.)
This word is destined to sweep the universe. If you don't like it, we don't care, so FLAGNOG it!!
If a hick named Anna whips out her hicky bracelet making supplies, Suzy exclaims "What the flagnog?!?" in her disbelief at Anna's hickiness.
If you are sleeping in history class and all of a sudden you feel your eye getting slapped, you sit up and yell "WHAT THE FLAGNOG?!?"
When you fail at something in life (eg. failing an exam or not getting a job), go home and spend an hour just standing in the shower, propping yourself up against the wall with your eyes on your feet.
Caleb knew the only thing to do after he didnt get prefect was take a failure shower.