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QM :"Daft as a brush" ? This was a catchphrase of Ken Platt, a Lancastrian 'flat cap' comedian, famous on radio in the 1950s. He had experimented with other versions of "'as a brush" the starting point being another "daft as.." , a local saying where the noun was a word of Lancastrian dialect , which he thought too obscure for a general audience.
So we do know exacty where "Daft as a brush" came from.
And now, as he used to say when opening his act, "Allo, I won't take me coat off. I'm not staying" :)