(someone) is going to flip out

To "flip out" means to temporarily become crazy with anger, fear, or excitement. It's usually used with negative emotions:

Why do you always flip out whenever I say anything even the slightest bit critical?

But you can also use "flip out" in positive situations meaning someone is becoming really excited:

Vince flipped out when I showed him the video quality of the new camera.

This phrase appears in these lessons: