or Grace-card. The six of hearts is so called in Kilkenny. At the Revolution in 1688, one of the family of Grace, of Courtstown, in Ireland, equipped at his own expense a regiment of foot and troop of horse, in the service of King James. William of Orange promised him high honours if he would join the new party, but the indignant baron wrote on a card, “Tell your master I despise his offer.” The card was the six of héarts, and hence the name.
It was a common practice till quite modern times to utilise playing-cards for directions, orders, and addresses.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894