That's my favourite idiom "As Daft as a Brush" so I thought I'd check it out. (That Don Marquis has got a lot to answer for - see previous posts!).
As Daft as a Brush
The Victorian chimney sweep was the source of this phrase.
The skilled child sweep would have to work his way vertically through the confined space of chimneys, clearing blockages and removing the build up of soot by hand.
Apprentice sweeps were merely dropped head first down the flue. This often lead to them being ‘lightly stunned’ or in some cases ‘severely cerebrally damaged’.
Hence the phrase “As Daft as a Brush”.
Incidentally, the high mortality of this practise lead to a shortage of chimney sweeps, and that’s why today it is purported to be lucky to meet a sweep.
Right now I've shown you mine - you must show me yours! 
That reminds me "As cool as a cucumber" - now there's another good one! 
ALL THIS FUN AND INFORMATIVE TOO! WHO SAID I WAS A FOOL?