grace-card, n. — Green’s Dictionary of Slang

grace-card n.

[‘A Kilkenny gentleman, named grace, being solicited, with promises of royal favour, to espouse the cause of William III, gave the following answer, written on the back of the six of hearts, to an emissary of Marshal Schomberg’s, who had been commissioned to make the proposal to him: ― “Tell your master I despise his offer; and that honour and conscience are dearer to a gentleman than all the wealth and titles a prince can bestow.”’ Hotten 1864, but adds in 1873: ‘This would have been a much better story had James II been a better King, and had he not earned for himself, even among Catholic Irishmen, a disgraceful name, through his craven conduct at the Battle of the Boyne’]

1. (Irish) in cards, the six of hearts.

1851		Durham Co. Advertiser 14 Nov. 2/1: Popular names for certain Playing Cards [...] Six of hearts...the grace card.
1864, 1867, 1870		Hotten Sl. Dict.
1873		Sl. Dict.

2. the ace of hearts.

1860

Hotten Dict. of Modern Sl. etc. (2nd edn).