Green’s Dictionary of Slang

a device, an expedient.

1763		F. Brooke Hist. of Lady J. Mandeville 31: Poor fellow! I pity him; but marriage is his only card.
1826		W. Scott Woodstock III 358: No card seemed to turn up favourable to the royal cause.
1853		G.J. Whyte-Melville Digby Grand (1890) 231: [of a horse] ‘Polly’s the card for you to stick to, ain’t she, Grand?’.