From early modern English *simper* (16th century), likely imitative of the soft, hissing sound of gently bubbling liquid. The spelling settled to *simmer* by the 18th century. The figurative sense — emotions simmering below the surface — emerged naturally from the idea of heat building without yet boiling over.

In culinary English, "simmer" is a precise instruction: the heat is lower than a boil (around 85–95 °C / 185–205 °F), producing gentle, lazy bubbles. Recipe writers treat it as distinct from "boil," "poach," and "steam." The figurative phrase *simmer down* — meaning to calm yourself after anger — is common in everyday American and British English (e.g., "Simmer down, everyone!").