Green’s Dictionary of Slang

1. to pose as a mute.

1926

J. Black You Can’t Win (2000) 69: I was dummyin’ up, see? imitatin’ a deaf an’ dumb man. D.D.ing, see?

2. to stop talking, to keep quiet.

1926		J. Black You Can’t Win (2000) 282: Red don’t see any use in givin’ himself a bawl-out by identifyin’ any dead burglars, so he dummies up.
1935		D. Lamson We Who Are About to Die 95: Dummy up; say nothing.
1943		W. Brandon ‘Its so Peaceful in the Country’ in Ruhm Hard-Boiled Detective (1977) 321: I dummy up. My plea is working on her.
1953		‘William Lee’ Junkie (1966) 97: If you dummy up on cops they will slap you around.
1969		‘Iceberg Slim’ Pimp 98: I dummied up.
1978		R. Caron Go-Boy! 57: When the lights went out [...] everybody was just supposed to dummy up.
1986		S. King It (1987) 525: Dummy up! All of you! Not one sound! Just dummy up!
1991		(con. 1920s) O.D. Brooks Legs 70: When everybody dummied up, he charged upstairs.
2003		J.V. Wemlinger A Path to Innocence, a Road to War 228: If he makes the bad choice to dummy up, I’ll need you to help me make him see the light.
2010		Codella and Bennett Alphaville (2011) 204: When one of them dummied up and refused to answer [...] we’d smack him in the head or across the mouth.

3. to keep something secret.

1951		D. Dressler Parole Chief 90: You see a guy cheatin’, you dummy up.
1955		‘Blackie’ Audett Rap Sheet 74: I dummied up and made out like they had grabbed the wrong guy. I said I was no more Blackie Audett than he was.
1961		M. Braly Felony Tank (1962) 15: Listen, kid, you better forget our business. If you’ve got any idea of living to get out of jail, you better dummy up on anything you hear in this cell.
1985		N. Pileggi Wiseguy (2001) 53: Because he dummied up and denied everything he never spent a day in jail.