intransitive verb. 1 : to become calm or peaceful. 2 : to become reduced by or as if by simmering.

Where does the term simmer down come from?

simmer (v.)

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]

What does simmer mean in slang?

Slang. to become calm or quiet, as from a state of anger or turmoil: We waited for the audience to simmer down.