Green’s Dictionary of Slang

to toss (as in coins).

1938		F.D. Sharpe Sharpe of the Flying Squad 278: Crooks are sometimes described by the fraternity as being ‘at the iron.’ This term is derived from the slang description of the Toss – ‘The Iron Horse.’ An Underworld man offering to toss for drinks will often say: ‘I’ll iron you for it’. [Ibid.] 331: iron horse: The toss (tossing with coins). Sometimes abbreviated to ‘the Iron,’ or ‘Ironing.’ (I’ll iron you for it’ – ‘I’ll toss you for it.’).
1972		Dodson & Saczek Dict. of Cockney Rhy. Sl.