E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Colours dyed in the wool or raw material before manufacture. In French, tendre en laine. Such colours are the most durable. We speak of a rogue ingrain, meaning one hopelessly bad. (In the grain, that is, in the texture.)
Tis ingrain, sir; twill endure wind and weather.Shakespeare: Twelfth Night, i. 5.