Green’s Dictionary of Slang

1. (US) meat balls.

1935

A.J. Pollock Und. Speaks.

2. rubbish, nonsense [balls n. (4)].

1943		M. Harrison Reported Safe Arrival 65: He shouted [...] ‘If you’re not out by then I’ll call the Captain to turn you out!’ ‘Niag’ra Falls!’ said Harry.
1978		Maledicta II:1+2 (Summer/Winter) 117: Place-names turned up [...] Niagara [Falls] = ‘balls’, but in the sense of UK rubbish and US bullshit, not ‘testicles’.

3. the testicles [balls n. (1)].

1960		J. Franklyn Dict. of Rhy. Sl.
1975		‘P.B. Yuill’ Hazell and the Three-card Trick (1977) 57: They were the same four who’d give you a quick boot in the Niagara Falls.
1985		R.G. Barrett You Wouldn’t Be Dead for Quids (1989) 122: I suppose I shouldn’t have reefed you in the Niagaras like that.
1998		Observer Rev. 9 Aug. 5 He jollied friends by kicking enemies in the ‘Niagaras’ and bare-knuckling them into oblivion.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 123/2: Niagara Falls n. testicles.
2002		D. Shaw ‘Dead Beard’ at www.asstr.org 🌐 While I was with him a hillman hunter stitched him up and Lingers used an Elsie Tanner on his Niagra falls. The poor bastard’ll never have another woman in his life.