/ɪnˈɡreɪn/
[verb] To fix a habit, belief, or feeling so deeply into a person's mind or character that it becomes very hard to change. You can also use it to describe something worked or dyed deeply into a material.
To 'ingrain' something was once to dye cloth so deep in the grain that no washing could ever lift the colour out.
The story of 'ingrain' begins not in the mind, but in the dye-house. In the textile trade of medieval Europe, fabrics were valued not just for their weave but for the permanence of their colour. Dyers discovered that certain scarlet and crimson dyes — derived from kermes insects, tiny creatures harvested from oak trees across the Mediterranean — produced a colour so deep and fast that it seemed to become part of the very substance of the cloth. This quality was described in Old French as 'en graine', where 'en' meant 'in' and 'graine' referred to the kermes dye, from Latin 'grana', meaning 'grain' or 'seed', because the dried insect bodies resembled small seeds or grains. So to dye something 'en graine' was to dye it 'in the grain' — into the very fibre and texture of the material.
When this phrase passed into Middle English during the 15th century, it came first as 'engrained' or 'in grain', always tied to this textile sense of a dye that had sunk so thoroughly into the cloth that it could never be removed. The Latin root 'grana' is worth pausing over: it gave English not only this word but also 'grain', 'garnet' (originally a deep-red gem the colour of pomegranate seeds), and 'grenade' (from Old French 'grenade' meaning pomegranate, because of its seed-filled interior). Every one of these words descends from the same ancient image of a small, hard seed.
By the 16th century, writers began using 'ingrained' in a figurative sense, moving the image from cloth to character. Just as a dye could be worked so deeply into a fabric that light and washing could not reach it, a feeling, habit, or belief could be worked so deeply into a person that reason and experience could not shift it. Shakespeare and his contemporaries used the word freely in this way — to speak of ingrained vice or ingrained loyalty was to say that these qualities were not surface features but had been fixed at the very core of a person's nature.
The spelling shifted over time between 'engrain' and 'ingrain', reflecting uncertainty about whether the prefix should be the French 'en-' or the Latin 'in-'. Both forms coexisted in English for centuries, with 'ingrain' gradually winning out in general use, though 'engrain' can still occasionally be found in older texts. The related adjective 'ingrained' became far more common in everyday speech than the verb itself — most people today would say a habit is 'ingrained' rather than say they 'ingrain' something, yet both forms are correct and in use.
What makes this word particularly rich is the physical honesty of its original metaphor. Medieval dyers understood that some colours were 'in grain' and others were merely surface tints that would fade with wear. When the word transferred to describe human psychology, it carried that same practical wisdom: some things in us are surface, and some things go all the way down. To call a prejudice or a kindness 'ingrained' is to say, quietly, that it has become indistinguishable from the fabric of the person who holds it.