A fool and his money are soon parted

A fool's paradise

A little knowledge is a dangerous thing

A load of cobblers

A load of codswallop

A man who is his own lawyer has a fool for a client

A no-brainer

Act the giddy goat

April fool

As daft as a brush

As mad as a hatter

As mad as a March hare

Ask a silly question and you'll get a silly answer

Away with the fairies

Barking mad

Bats in the belfry

Cock-up

Daft as a brush

Don't have a clue

Fools rush in where angels fear to tread

Fools' gold

Giddy goat

Go Dolally

Go haywire

Go postal

Lark about

Like a chicken with its head cut off

Lose your marbles

My giddy aunt

Not playing with a full deck

One sandwich short of a picnic

Play the giddy goat

Pond life

They couldn't hit an elephant at this dist...