Enmity is a formal word that refers to a very deep unfriendly feeling, such as hatred or ill will, that is often felt mutually.

// Having to collaborate on the project only increased the bitter enmity between the two coworkers, who had never gotten along.

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"Paul Monreal is a fourth-great-grandchild of Catherine and Patrick O'Leary, who endured the enmity of Chicagoans after they were wrongfully accused of starting the Great Chicago Fire, which legend said was started by a jittery dairy cow named Daisy." — William Lee, The Chicago Tribune, 5 July 2025

The resemblance between enmity and enemy is no coincidence: both words come from the Anglo-French word enemi, which literally translates to "enemy." And when you feel enmity for a particular person—that is, deep-seated dislike or ill will—"enemy" may very well be an apt descriptor for them. While it is possible to feel enmity for someone who does not share or return one’s animosity, enmity is typically used for mutual hatred or antagonism between people (or groups, factions, etc.), as when Edgar Allan Poe wrote of the families of Berlifitzing and Metzengerstein in his first published short story: "Never before were two houses so illustrious, mutually embittered by hostility so deadly. The origin of this enmity seems to be found in the words of an ancient prophecy—'A lofty name shall have a fearful fall ...'"