It’s not that simple. Both forms have appeared for centuries.
The image of a card shark conjures up a predatory player who will rip you to shreds like something out of Jaws. Unfortunately, the original shark was not a fish at all. It is probable that it was a variant on a German word, schurke, a scoundrel or villain. It shows up in Ben Jonson’s work of 1599, The Comicall Satyre of Every Man Out of His Humor. In that work, he describes a shark, a swindling beggar, who pretends to be a former soldier down on his luck who must resort to abject begging. Jonson describes him as “One that neuer was Soldior, yet liues vpon lendings.” By the 18th century, the image of a scoundrel had merged with the image of a preying shark, the fish.
Then there’s a sharper, which the Oxford English Dictionary defines as “A cheat, swindler, rogue; one who lives by his wits and by taking advantage of the simplicity of others; esp. a fraudulent gamester.” The first instance given is from 1681: “Many of them sharpers about town.” [Narcissus Luttrel, A brief historical relation of state affairs 1678–1714.] When it was shortened to a sharp, in 1797, it was probably because a swindler had to be sharp-witted, clever, and edgy.
Card sharp as a construction shows up in Bret Harte’s On Frontier , while card shark (in conjunction) doesn’t get recorded until the 1940s.
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