simmer down

Verb

Meaning

become quiet or calm, especially after a state of agitation

Source: WordNet

chill out calm settle down calm down cool off cool it cool compose relax chill unwind take it easy lighten up decompress loosen up

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fume lose it rage explode flip boil

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turn change state

After the fight both men need to cool off. Source: Internet

It took a while after the baby was born for things to settle down again. Source: Internet