Green’s Dictionary of Slang

shtoom adj.

quiet, silent, dumb.

1956		J.L. Robinson Silence is Goldberg 63: ‘You ain’t got the brains to be a Yiddishe parrot [...] I bring in the boys an’ I take bets on ya — an’ whaddya do? Ya go shtoom on me!’.
1962		F. Norman Guntz 71: I didn’t feel like going into it just then so I stayed shtoom.
1962		R. Cook Crust on its Uppers 105: There wasn’t anything to be said, so we [...] stayed stumm.
1975		(con. 1939) J. Rosenthal Evacuees Scene 23: So have you both gone stumm or something?
1989		J. Morton Lowspeak 135: Stumm – to remain silent.
1999		Guardian Weekend 18 Sept. 43: The audience goes shtumm, nothing, absolute silence.
2000		Daily Tel. 10 Jan. 19: She can stay stumm and work on him, Henry Higgins-like, behind the scenes. She can walk out.
2009		J. Joso Soothing Music for Stray Cats 139: The trick would be to stay schtum.
2024		J. Meades Empty Wigs (t/s) 162: Wells, however, kept his counsel. He stayed schtum.

In phrases

keep shtoom (v.) (also keep schtum, …stum(m), …stumpf)

to keep quiet, to say nothing.

1957		L. Wilkinson Behind the Face of Crime 44: He explained that ‘schtum’ was a Yiddish word for ‘quiet’.
1958		F. Norman Bang to Rights 15: I think it’s much better to keep shtoom.
1974		L. Deighton Spy Story 41: So far, both sides have kept stumm about these operations.
1978		F. Norman Dead Butler Caper 84: I kept shtoom.
1982		J. Sullivan ‘It Never Rains’ Only Fools and Horses [TV script] I told you keep schtum!
1983		A. Payne ‘Get Daley!’ Minder [TV script] 29: Would I keep stum over something like this?
1985		S. Berkoff West in Decadence and Other Plays (1985) 112: I might carry in my heart / a little doubt / so I’ll keep shtoom / and render him my total confidence.
1986		R.G. Barrett Godson 197: ‘Keep schtoom. Say nuffink to no one’.
1989		T. Blacker Fixx 166: There’s a time for publicity, a time for keeping schtum.
1989		S. Armitage ‘Eighties, Nineties’ in Zoom 60: One William Creamstick / who was keeping shtum on the Scottish borders.
1997		N. Barlay Curvy Lovebox 95: Ellyu keeps stumm ignorin’ him.
1997		R. Barnard No Place of Safety 173: Unless he had some reason for keeping shtoom that outweighed the danger.
1998		I. Welsh Filth 49: The best way tae handle these cunts is just tae keep stumpf.
1999		Guardian Rev. 22 Oct. 15: He’s keeping shtoom about that for now.
1999		Indep. Rev. 10 June 8: My advice would be to keep shtum.
1999		Indep. on Sun. Real Life 27 June 2: Try the silent meal [...] eat studiously and keep schtum.
1999		J. Cameron Brown Bread in Wengen [ebook] ‘You want to keep it stum though it’s best you button it’.
2000		Indep. Rev. 8 June 11: Keep shtoom.
2000		J.J. Connolly Layer Cake 7: Morty kept schtum and did his time.
2002		J. Cameron Hell on Hoe Street 127: I kept stum.
2002		D. Shaw ‘Dead Beard’ at www.asstr.org 🌐 Anyway, are you going to keep stum about clocking me here, or are you going to tell Lingers?
2005		B. Hare Urban Grimshaw 94: They all agreed to keep shtum.
2012		(con. 1980s) I. Welsh Skagboys 174: She’s keepin stumpf, noddin in some kind ay sympathy.
2016		Guardian 5 Dec. 🌐 This was an era when the [Jewish] community’s leaders adopted a ‘keep stumm’ policy – heads down, don’t cause a fuss, don’t rock the boat.
2016		V. McDermid Out of Bounds (2017) 401: They might have had their own reasons for keeping shtum.

shtoom up (v.) (also stumm up)

to be quiet.

1958		F. Norman Bang To Rights 72: You can always shtoom up if any screws are earholeing.
1962		F. Norman Guntz 90: This confused Fred so he shtoomed up.
1979		F. Norman Too Many Crooks Spoil the Caper 138: She got the message and shtoomed up.
2001		N. Barlay Hooky Gear 119: I’m waitin for him, kind of stumm up by his words like he slap me too.
2019		‘Interiewing Ira Glass’ in https://kottke.org 🌐 [H]e kind of schtums up and falls back on generalities and a few broad compliments. I don’t know. Maybe that's all he’s got.