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Origin documented
“A grain of truth”
What it means
A very small amount of accuracy in something largely false.
Where it comes from
The grain is the smallest unit in the British weight system, at 64.79891 milligrams, and it is identical across avoirdupois, troy and apothecary reckoning. Nothing in the system is smaller.
How sure are we?
Literal in origin, from the weight of a single grain of barley. The figurative use of the smallest possible quantity is common to many languages.