Green’s Dictionary of Slang

1. (US Und.) a pistol with a silencer.

1944		‘Jiver’s Bible’ in D. Burley Orig. Hbk of Harlem Jive.
1950		Goldin et al. DAUL 104/2: Hush-hush. A gun with a silencer; a dumb-gat; a sissy rod.

2. secrecy.

1945		J.B. Priestly Three Men in New Suits 85: More ‘off the record’ talk [...] more global ‘hush-hush’.
1953		J. Thompson Criminal (1993) 51: There’s been too much hush-hush about these juvenile criminals.
1995		L. Stavsky et al. A2Z 59/2: keep it on the hush-hush – v. to tell no one.
2008		Simon & Burns ‘Transitions’ Wire ser. 5 ep. 4 [TV script] I know you all want to keep things on the hush-hush tip.
2012		L. Berney Whiplash River [ebook] Babb wasn’t supposed to know who that was—hush-hush—but of course he knew.

3. (US) a secret.

1965

(con. 1940s) Malcolm X Autobiog. (1968) 176: She started hearing [...] the hush-hush that ‘niggers’ were such sexual giants and athletes.