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. 241-244.
Rogue in grain. A punning application of the above phrase to millers. To go against the grain. Against one's 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 meaning—thus, “It le mangcrait 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 lætitia, joyfully, cum grano, minutely (“cum grano salis, ” in the minute manner that one takes salt).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894