The act of declaring that one is good at something right before fucking it up.
Derives from Star Wars Episode III: Revenge of the Sith where Anakin Skywalker says this to his former master Obi-Wan Kenobi right before lunging at him and getting his left arm and both of his legs cut of by a lightsaber.
Don't underestimate the funny/silly looking guy/girl trying to disarm you, for he/she can go from funny/silly to serious and grown looking/sounding like a light switch, and bad things happen to people that underestimate him/her. His/her bag of tricks is deep, and they can all die.