Green’s Dictionary of Slang

1. (US gambling) a bet that it is hoped will recoup earlier losses.

1950		‘Josephine Tey’ To Love and Be Wise 216: ‘No bets on the field?’ ‘N-o. I think not. A saver on Weekley, perhaps’.
1981		D. Jenkins Baja Oklahoma 222: ‘I like the Over on Arkansas-Rice. I’m teasing the Aggies with Alabama and Army. Oregon’s the West Coast saver’.

2. (UK und.) in a swindle, a sum of money set side so one can at least pay smaller sums that are owing to the clients.

1953

J. Phelan Underworld 54: The saver is very important. Some swindles need three or more savers, but all need at least one.