Here are some more sayings

Not enough brains to give 'imself a headache!

Off like a bride's nightie.
Very quickly.

About as useful as **** on a bull.
~explains itself

Happy as a ******* on Father's Day.
~ explains itself

Got you by the short and curlies.
Got you on a short leash.... pubic hairs!

You must be the world's only living brain donor.
dumb or stupid to the extreme

The most fun you can have with your pants on.
~ explains itself

A few wanks short of an orgasm.
not all there

Busier than a one armed Sydney cab driver with the crabs.
~ explains itself