Wiktionary
vb. (context intransitive idiomatic English) To decrease in intensity of anger, agitation, or excitement.
WordNet
Wikipedia
"Simmer Down" was the first single released by The Wailers, accompanied by the ska supergroup, The Skatalites, and produced by Clement "Coxsone" Dodd in 1963. It was the number one hit in Jamaica in February, 1964.
The song was directed to the " Rude Boys" of the ghettos of Jamaica at the time, sending them a message to cool down or "Simmer Down" with all the violence and crime going on in Kingston. The subject matter of "Simmer Down" made The Wailers stand out amongst their contemporaries. The Wailers at this time contained Bob Marley, Bunny Wailer, Peter Tosh, Junior Braithwaite, Cherry Smith, and Beverley Kelso. It was Bob Marley's first hit and his career as a songwriter and performer took off from there.
Although "Simmer Down" was a hit, Peter Tosh, one of the three original Wailers, has said in an interview that he hated it.
Later, the song was covered by The Mighty Mighty Bosstones for their EP Ska-Core, the Devil, and More, as well as by The Specials for their cover album Today's Specials.
Usage examples of "simmer down".
Place in 2 quarts of water and gently simmer down to 1 quart, strain and take a wineglassful every two hours.
So he waits a little for Cox to simmer down, and then he comes out, asking for mercy when Cox raises up the gun.
Closed up the apartment and took a holiday until the whole dirty business has a chance to simmer down?