To foment something, such as hostility or opposition, is to cause it, or try to cause it, to grow or develop. Foment is used synonymously with incite.

// Rumors that the will was a fake fomented distrust between the two families.

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"For this prequel to The Witcher, we go back, back, back to 1,200 years before the time of Geralt of Rivia—and if you don’t know who that is, it matters not. Slide right into the self-contained story of a continent where elves, dwarves and other often-warring peoples are living in uneasy proximity, until the arrival of one vicious dictatorship to rule them all makes everyone even less relaxed. Out in the sticks, soldier turned travelling bard Éile (Sophia Brown) is already fomenting revolutionary solidarity by singing rousing folk songs in pubs…" — Jack Seale, The Guardian (London), 25 Dec. 2022

If you had sore muscles in the 1600s, your doctor might have advised you to foment the injury, perhaps with heated lotions or warm wax. Does this sound like an odd prescription? It's less so if you know that foment traces to the Latin verb fovēre, which means "to heat or warm" or "to soothe." The earliest documented English uses of foment appear in medical texts offering advice on how to soothe various aches and pains by the application of moist heat. In time, the idea of applying heat became a metaphor for stimulating or rousing to action. Foment then started being used in political contexts to mean "to stir up" or "to call to action."