That feeling you get when you feel like you accomplished something important, but then soon realized you either didn't need to do it, or there was no result
You stab him in the kidney. You just killed the murderer Shia LaBeouf!
But wait he's not dead shiasurprise! There's a gun to your head, and death in his eyes.
Short-hand for the word floccinaucinihilipilification, meaning the estimation of something as valueless (encountered mainly as an example of one of the longest words in the English language).