[Q] From Jeremy Redgrove in Australia: Do you know the origin of the expression chalk and cheese? I heard that it came from two neighbouring counties (I think Devon and Dorset) and their contrasting products of chalk and cheese.
[A] As different as chalk and cheese is an old proverbial phrase to suggest that two things, superficially alike, are totally different in their qualities. There’s nothing in its history to suggest these two counties had anything to do with it — it sounds like yet another folk etymology to me.
The earliest example — from John Gower’s Confessio Amantis of 1393 — suggests that some shopkeeper was making an illicit profit by adulterating his wares: “And thus ful ofte chalk for cheese he changeth with ful littel cost”. The buyer was surely undiscerning; though some British cheeses are rather chalk-like in appearance, substituting more than a tiny proportion of cheese with chalk wouldn’t fool anybody for very long.
By the sixteenth century, the phrase had become a fixed expression. Hugh Latimer wrote rather sarcastically around 1555: “As though I could not discern cheese from chalk.”