- 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…