Mad as a cut snake
Furious, or completely crazy.
Person 1"Did you tell Kev about the ute?"
Person 2"Yep. He's mad as a cut snake."
You'll also hear
Cross as a frog in a sockangry
Mad as a hatterBritish version
Where it's from
A snake cut in half keeps writhing and striking.

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While you're here
Crazy, or very angry.
Extremely busy. Can be sarcastic about someone who isn't busy at all.
Extremely thirsty. Also used for weather that has not seen rain in months.
Completely full, usually after a big feed. Sometimes used for very drunk.
Showing off and very pleased with yourself.
Almost impossible to find.