The colloquial British-English expression (as) daft as a brush means extremely stupid, very silly—cf. also, in particular, the similes mad as a hatter and mad as a March hare, and ‘a sandwich short of a picnic’ and other phrases meaning ‘stupid’ or ‘crazy’.
This expression seems to have originated in Devon, a county of southwestern England, as (as) mazed as a brish [= brush].
According to Alexander Games in Kick the Bucket and Swing the Cat: The Balderdash & Piffle Collection of English Words, and Their Curious Origins (BBC Books – London, 2008), (as) mazed as a brish is first recorded in The Labouring Life (Jonathan Cape – London, 1932), a book set in a Devon village, by the English author Henry Williamson (1895-1977); Chapter 1, “being samples of the sayings of the village”, contains the following:
I have, however, discovered a much earlier instance of the phrase in a letter tu tha Hedditur uf tha “Vlying Pawst” [= to the Editor of the “Flying Post”], titled Tha Haggerikulterul Laberer [= The Agricultural Labourer], by a person signing themself Radger Geales, published in Trewman’s Exeter Flying Post (Exeter, Devon) of Saturday 18th June 1892:
Mister Hedditer, wen u zee Zusan in the Zitty, pleaze ta zay us all zend oure kind luv. Her liv’th zomeware near tha Katheyderall, an her’s za vine a maid es evver stepped in shoo leather, and Walter Axford iz za mazed es a brish arter Zusan.
in standard English:
Mister Editor, when you see Susan in the City, please say we all send our kind love. She lives somewhere near the Cathedral, and she is as fine a maid as ever stepped in shoe leather, and Walter Axford [= Ackford?] is as mazed as a brush for Susan.
The second-earliest instance of (as) mazed as a brish that I have found is from an account of the annual meeting of the Devonshire Association for the Advancement of Science, Literature and Art, held at Okehampton, Devon—account published in The Western Morning News and Western Daily Mercury (Plymouth and Exeter, Devon) of Thursday 23rd July 1925:
“Devonshire Verbal Provincialisms,” dealt with in the 38th report, was the subject of a paper by Mr. C. H. Laycock, some of the examples being, “Beaver, or bever”—lunch time; “breathing air”—native air; “brish”—brush, as “Mazed as a brish,” a common saying around Newton Abbot; [&c.].
The following day, Friday 24th July 1925, The Devon and Exeter Gazette (Exeter, Devon) added the following details about the paper on “Devon Verbal Provincialisms” given by Mr. C. H. Laycock:
The earliest instance of (as) daft as a brush that I have found is from Murder Threats To Girl Alleged: Lincoln Man’s Letters Read In Court, published in the Lincolnshire Echo (Lincoln, Lincolnshire) of Wednesday 21st June 1944; this article gave an account of the trial of George Burbridge, a 22-year-old Lincoln foundry worker, “accused of writing letters threatening murder to a girl with whom he had been keeping company”:
In the letter to the father, received on May 12, Burbridge said he was hoping to return to Lincoln—(he was then working in Manchester)—but added,
“Please do not think I am coming to see your daughter because it is out of my mind even to look at her again . . . Your daughter told me she did not want me the day before I left . . . I will agree with anything that says I am daft, daft as brush, but I have got enough sense to keep away from your daughter.”
This paragraph, dated 12th February, was written from Keswick, then in Cumberland, a former county of northwestern England; according to “the general opinion” expressed in this text, the origin of daft as a brush, but also of daft as a swill [= a big basket], daft as a gate and daft as a wagon-horse, is that anything is daft that does all the hard work—interestingly, this could also account for the above-mentioned Devon phrase mazed as a broomstick:
It all began over Champ the foxhound: he is in his tenth winter with the local pack and evidently feels that the time has come for him to pick his hunting days and so, finding himself one day lately near the farm where he spends his summers, he left the other dogs to carry on and dropped in to see what there was to eat. There was a tremendous buffet at the door and in he came “laughing his head off,” as the farmer’s wife said. He really does laugh, lifting his lip up off his teeth, shaking his head to one side with joy, and giving everyone the full benefit of his pleasure. “Daft as a swill,” she called him and that did it—how daft can one be? There are many variants—“daft as a swill” (a big basket), “daft as a yat” (gate), “daft as a brush,” and “daft as a wagon-horse.” At first it is difficult to see any connection between them all, but it is very simple, really. “Owt is daft ’at does all t’ hard work” was the general opinion. “What carries all t’ heavy loads? T’ swill. T’ yat’s for ivver being opened an’ shut an’ if you want to deu owt in t’ yard t’ brush gets all t’ mucky jobs. An’ as for t’ wagon-horse—think of all t’ rivin’ an’ pullin’ an’ sniggin’ [clearing small timber] it does—it mun be daft t’ deu all that.” There are, however, degrees of daftness and the farmer had the last word who said: “All t’ nicest fowk are a laal bit daft any road.