Simmer Down Meaning
(intransitive, idiomatic) To decrease in intensity of anger, agitation, or excitement.
Example: 1870, Mark Twain, "Goldsmith's Friend Abroad Again," letter 4:
"Silence! Now ye had better go slow, my good fellow. This is two or three times you've tried to get off some of your d----d insolence. Lip won't do here. You've got to simmer down."
1910, Stewart Edward White, The Rules of the Game, ch. 73:
The agitation, thus deprived of its chief hope, might very well have been expected to simmer down, to die away slowly.
2003 June 30, Nazila Fathi, "British Minister Presses Iran To Allow Nuclear Inspections," New York Times (retrieved 9 Apr 2009):
Although the street demonstrations have simmered down, protests have continued in other forms.