Green’s Dictionary of Slang

1. (Aus.) to give in [? poss. mis-defining of sense 3].

1882

Sydney Sl. Dict. (2 edn) 2: Clam - To give in.

2. to refuse food.

1887

W.E. Henley ‘Villon’s Good-Night’ in Farmer Musa Pedestris (1896) 175: And gave me mumps and mulligrubs / With skilly and swill that made me clam.

3. (also do the clam) to stop talking, to become deliberately secretive; thus as the clam-ups n., silence, a refusal to speak.

[1838		Crim.-Con. Gaz. 22 Sept. 40/1: I couldn’t sqaeel [sic], for I was out of wind, and my voice was clamm’d till he let me up].
1915		D. Lowrie My Life out of Prison 285: I got one of the other reporters to do it, because [...] if S—— learned that I was talking to him he might ‘clam’.
1916		H.L. Wilson Somewhere in Red Gap 237: When I ask for details he just clams up.
1925		D. Hammett ‘Tom, Dick, or Harry’ in Nightmare Town (2001) 243: He’s doing the clam now; he hasn’t said a word.
1926		J. Black You Can’t Win (2000) 87: Just clam up, kid. Tell them you’d rather not say anything till you get a lawyer.
1929		Hostetter & Beesley It’s a Racket! 222: clam—To keep absolutely quiet; to refuse to talk.
1935		R. Chandler ‘Nevada Gas’ in Spanish Blood (1946) 161: With Parisi dead he’d clam up.
1941		J.K. Butler ‘Saint in Silver’ in Goulart (1965) (1967) 81: If you want to clam up, then clam.
1952		L. Lariar Day I Died 116: Now be a good kid and clam up, will you. I need some shut-eye.
1963		G. Lambert Inside Daisy Clover (1966) 179: Then people think they’ve gone too far and clam up.
1977		S. Stallone Paradise Alley (1978) 55: Clam up, I’m talkin’.
1983		C. Heath A-Team 2 Again she clammed up.
1996		(con. 1964-65) B. Thorpe Sex and Thugs and Rock ’n’ Roll 334: ‘If she sees you coming she’ll clam up tighter than a crab’s arse’.
1998		Guardian Guide 25–31 July 18: Others reveal as much by clamming up.
2001		J. Stahl Plainclothes Naked (2002) 189: I […] was knocking around with a hot chick who probably offed her husband. I’d get the clam-ups, too.
2004		E. Weiner Drop Dead, My Lovely (2005) 135: He was threatening to clam up on me.
2005		P. Temple Broken Shore (2007) [ebook] One bloke [...] raved on, then he clammed up.
2010		(con. 1973) C. Stella Johnny Porno 244: Vento may or may not clam up.
2017		A. Parks Bloody January 44: She chewed on her bottom lip [...] unsure whether to go on or not. [...] McCoy thought she was going to clam up.
2017		L. McInerney Blood Miracles 95: ‘[T]he boys abroad, they’re not going to be impressed if we suddenly clam up’.
2020		D. Winslow ‘The San Diego Zoo’ in Broken 162: ‘Why did you do that, Hollis?’ Hollis clams up.
2023		R. Milward Man-Eating Typewriter 113: [H]e’d clam up whenever I tried to [...] pester him.