Word Origin & History
1650s, alteration of simperen "to simmer" (late 15c.), possibly of imitative origin. Figurative sense, of feelings, "to be agitated" is from 1764. Opposite sense, in simmer down, first recorded 1871.
"I must and will keep shady and quiet till Bret Harte simmers down a little." [Mark Twain, letter, 1871]