It is American slang for "extremely crazy". "--adjective-- as all get out" is largely heard in the US South. It is an idiom that can't be broken down into parts.

This phrase doesn't make sense.

Yes, it means "extremely crazy", but it's not a common idiom. It's an Americanism, so don't expect every native English speaker to know or accept it.

I agree with Peachey. This might be a useful phrase to understand if you are living in the southern states of the USA. Otherwise, it's not much use to you. I'm sure you have the same situation in China. Some expressions might be understood anywhere, while others are restricted to a comparatively small area and are meaningless to anyone else.