Silly as a two-bob watch
Daft or erratic.
Person 1"Dazza wore a tuxedo to the barbie."
Person 2"Silly as a two-bob watch."
You'll also hear
Mad as a two-bob watchsame
Where it's from
Two bob was two shillings. A watch that cheap would not keep good time.

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While you're here
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.
Extremely happy.