Green’s Dictionary of Slang

1. a gun.

1920		H. Wiley Wildcat 244: ‘Always go loaded for bear, I suppose?’ ‘Sho’ do. I aims to pack a li’l equalizer all de time.’.
1938		W. Pegler in Nevada State Journal (Reno) 21 Dec. 6/2: That was long before [a gun] was called a rod, a term which gave way to the more poetic form of equalizer, which, in turn, was challenged by the word manhood.
1942		R.L. Bellem ‘Malibu Mess’ Dan Turner – Hollywood Detective Dec. 🌐 ‘You won’t get away with it, though. I’ll—’ He raised his equalizer.
1953		A.J. Liebling Honest Rainmaker (1991) 149: He was more on the order of Al Jennings the train robber [...] only he didn’t need an equalizer.
1965		J.P. Carstairs Concrete Kimono 100: ‘Well, if you haven’t a deringer—’ ‘A what?’ ‘An equalizer, a gun—’.
1989		J. Morton Lowspeak.
1990		M. McAlary Crack War (1991) 47: He carried the great equalizer, a .38 cal. Smith & Wesson.
2003		M. Pryce Last Tango in Aberystwyth 124: ‘A heater?’ ‘Protection ... an equaliser.’.

2. any weapon, a cosh, a knife, a bomb.

1899		A.C. Gunter M.S. Bradford Special 281: ‘Fifty t’ousand dollars, or I throws dis Equalizer at your foots, and blow you to nottingness!’ [...] ‘A bomb? It will blow me up?’ she gasps.
1943		Crowley & Sachs Follow the Leader [film script] Oh, that’s the old equalizer [i.e. a blackjack].
1954		Wodehouse Jeeves and the Feudal Spirit 153: I reached a hand into my pocket and got a firm grasp on the old Equalizer.
1958		E. Aarons Gang Rumble (2021) 8: The knife was the old equalizer, all right.
1969		‘Iceberg Slim’ Mama Black Widow 179: I was looking for a knife or an equalizer of some kind.
2004		(con. 1960s-70s) T. Taylor Top Fellas 26/1: A fence-post made for a nifty equaliser.