E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A knave in grain. A knave, though a rich man, or magnate. Grain means scarlet (Latin, granum, the coccus, or scarlet dye).
A military vest of purple flowed
Livelier than Melibean [Thessalian], or the grain
Of Sarra [Tyre] worn by kings and heroes old
In time of truce.
Paradise Lost, xi. 241244.
Rogue in grain. A punning application of the above phrase to millers.
To go against the grain. Against ones inclination. The allusion is to wood, which cannot be easily planed the wrong way of the grain.
With a grain of salt. Latin, Cum grano salis, with great reservation. The French phrase has another meaningthus, Il le mangerait avec un grain de sel means, he could double up such a little whipper-snapper as easily as one could swallow a grain of salt. In the Latin phrase cum does not mean with on together with, but it adverbialises the noun, as cum fide, faithfully, cum silentio, silently, cum ltitia, joyfully, cum grano, minutely (cum grano salis, in the minute manner that one takes salt).