The ♦9 was first referred to as The Curse of Scotland in print in 1710. There are several explanations for the origin of the term, none completely authoritative.
The most likely explanation is that it refers to Sir John Dalrymple, the first Earl of Stair, who died in 1707. The Dalrymple family crest and the pattern on the ♦9 are strikingly similar.
Dalrymple was an effective politician, but ruthless in his pursuit of causes that were deeply unpopular with many Scots. He was especially loathed for his connection with the Glencoe Massacre (1692) and the Act of Union with England (1707).
An alternate theory revolves around a card game called Pope Joan. The ♦9 was called the Pope, the antichrist of Scottish reformers. A variant of this theory is that the ♦9 was the chief card in the game cornette, introduced into Scotland by the unhappy Queen Mary.
Other unlikely theories include that the link resulted from every ninth king of Scotland having been a tyrant and a curse to the country. Apparently the proportion of bad Scottish kings was much higher than one in nine. Or that the link stems from the theft of the nine diamonds from the Crown of Scotland and the tax the monarchy enacted to pay for their replacement.
The most unlikely theory, based on the date, is that the Butcher of Cumberland wrote the orders of “no quarter” for the Battle of Culloden, 1746, on the back of the ♦9.