Green’s Dictionary of Slang

to calm down.

1868		Irishman (Dublin) 4 Apr. 13/1: Thus disappointed, he ‘simmered down’ to an [sic] humble pleading for constitutional rights.
1871		Lincs. Chron. 17 Feb. 6/1: When a policeman came on the scene [...] he ‘simmered down’.
1877		Chicago Trib. 7 Apr. 3/1: Simmer down — you’ve got ‘too much brass’.
1874		‘English Sl.’ in Eve. Telegram (N.Y.) 9 Dec. 1/5: Let us present a few specimens:– [...] ‘Simmer down.’.
1899		Lancs. Eve. Post 4 Nov. 2/7: Birchall suffered from an ‘exuberance of animal spirits’ [...] he would find referees a trifle unsympathetic [...] unless he ‘simmered down’.
1900		Marvel 21 Dec. 2: Simmer down, Ephraim – simmer down!
1904		Buffalo Eve. News (NY) Untold blessings on the man / Telling people how theh can / With a smile accept a plan / To simmer down: .
1907		Derby Dly Teleg. 4 May 2/5: When he reached the people’s chamber he simmered down.
1912		London Dly News 29 Jan. 8/4: Brooks was never mastered, but he simmered down a little in the second half.
1925		Wodehouse Carry on, Jeeves 56: If my principles would permit it, I would simmer down for your sake.
1939		Sunderland Dly Echo 28 Feb. 7/3: He simmered down with surprising alacrity.
1949		Sentinel (Carlisle, PA) 19 Aug. 5/3: Don’t let your angry passions rise, / Just gently simmer down.
1957		Medford Mail Trib. (OR) 29 Dec. 4/2: [headline] ‘Simmer Down, Simmer Down’ Uncle Sam’s middle name is not Moderation.
1965		W.C. Anderson Adam M-1 109: Simmer down, old man.
1976		Times Record (Troy, NY) 30 Mar. 6/1: The public should step in, right now, and shout a loud ‘Simmer down’.
1987		W.D. Myers Crystal 40: ‘Okay, class, let’s simmer down’, .
1990		Eble Campus Sl. Apr. 7: simmer down – calm down.
1997–2000		Da Bomb 🌐 25: Simmer: Calm down. Hey, simmer down; you ‘re wiggin’ out.
2010		Index-Jrnl (Greenwood, SC) 18 Dec. 7/5: I feel permission to gear down, simmer down and quiet dowen.