simmer v.

"make a gentle hissing sound, as liquids when they begin to boil," hence "become heated gradually," especially of liquids kept just below the boiling point, 1650s, simber (the modern form is from 1680s), an alteration of simperen (late 15c., simpring "simmering," in an alchemical text), which might be imitative and is not thought to be a relative of simper (v.). OED says the sound change in this word is "probably due to a feeling of phonetic appropriateness."

The figurative sense, in reference to feelings, "be agitated or ready to boil over, be in a state of repressed excitement," is by 1764. The opposite figurative sense, in simmer down, is recorded by 1848, perhaps from the notion of reduction or moving from a full boil to a mere simmer. The literal sense of simmer down is attested from 1803, in instructions in cookery and herbal preparation. Related: Simmered; simmering. The noun meaning "a condition of simmering, a gentle, gradual, uniform heating" is by 1793 (on the simmer).