The Concise New Partridge Dictionary of Slang and Unconventional

slots · coin machine · coin-operated machine · fruit machine · gaming machine · one-armed bandit

It's referring to the 50p coin not being round I remember it being an odd shape, it's a heptagon Reply Kevin Gregg says:

coin slot meaning coin machine · coin-operated machine · fruit machine · gaming machine · one-armed bandit · slots

coin slot Coin slot — can refer to: A coin receptacle on a vending machine A slang term for buttock cleavage This disambiguation page lists articles associated with the