Daft as a brush...

When you poke with a wet brush you dab.. When you poke with a dry brush you "daff". When a brush has been "daffed" a lot its bristles splay out in all directions.  The brush is then called "daft" and it becomes practically useless...
Hence my favourite explanation of the etymology of "daft as a brush" meaning ..."useless"....or a person unable to direct their attention properly to the matter in hand.

Why blip a daft brush today... well because I was using it to wet the wall I was pointing...and I dafted it pretty well..