Simmer
mid 17th century: alteration of dialect simper (in the same sense), perhaps imitative.
wiktionary
From alteration of dialectal simper, from Middle English simperen(“to simmer”)[late 15th c.], of possibly imitative origin. First attested in the intransitive sense. The noun is from the verb.
From sim(“simulation”, noun) + -er.
etymonline
simmer (v.)
1650s, alteration of simperen "to simmer" (late 15c.), possibly imitative; not thought to be connected to simper (v.). OED says the change is "probably due to a feeling of phonetic appropriateness." Figurative sense, of feelings, "to be agitated" is from 1764. Opposite sense, in simmer down, first recorded 1871, probably from the notion of moving from a full boil to a mere simmer.
I must and will keep shady and quiet till Bret Harte simmers down a little. ["Mark Twain," letter, 1871]
Related: Simmered; simmering. The noun meaning "a condition of simmering" is from 1809.
