When something or someone causes you to lose your pleasure or ruins your enjoyment. Going from happy or laughing to immediately sad or depressed.
The teacher's homework was a total erection deflection. Taylor has been a real erection deflection lately. Enos, why are you such an erection deflection? Oh man your girlfriend is cheating on you, what an erection deflection.
The state of matter in which you are called up to the board in class with an erection. In a last ditch effort to deflect that which is erect you offer a means of distraction, such as a brick through a window or a pair of inflatable fake boobs.
I was able to pull of the erection deflection in my english class.