Daft as a brush
Definition 1 of 1
Pro Tip 1/3British Origin
This phrase is particularly used in the UK and might not be understood elsewhere. 
When I said he was 'as daft as a brush', my American friend was confused.
Pro Tip 2/3Humorous Tone
Used for light-hearted criticism or teasing, not meant to be hurtful. 
You're as daft as a brush, wearing shorts in the snow, but it's funny!
Pro Tip 3/3Informal Use
It's an informal idiom, not suited for formal writing or speeches. 
In a casual chat, I might say my mate is 'as daft as a brush'.