simmer down is a verb meaning "become quiet or calm, especially after a state of agitation". It carries one recorded sense.
become quiet or calm, especially after a state of agitation
“After the fight both men need to cool off.”
“It took a while after the baby was born for things to settle down again.”
Definitions, synonyms and semantic relations for simmer down come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.