A card sharp is someone who makes money playing card games such as poker.

A sharp is an expert, but the noun phrase card sharp means not just an expert, but a cheat. The phrase is also spelled card-sharp and cardsharp. In fact, there's a sixteenth-century painting by the Italian painter Caravaggio called "Cardsharps." In the painting, a card player is so engrossed in the game that he doesn't realize that another man is sneaking a peek at his cards and signalling to the card sharp, another player who has extra cards secretly tucked into his pants. The card sharp is going to cheat to win.

These examples get it right:

Gold's industry peers liken the way she does her job to the practice of an alchemist, a snake charmer, a card sharp, as if she were performing some kind of shadowy magic. (The Guardian)

A nervous twitch or other "tell" can alert a card sharp to what an opponent may be holding. (Washington Times)

Card sharp is correct, but seeing it spelled card shark is common. In addition to being a sharp-toothed marine animal, the word shark has been used to mean "a dishonest person who preys on others" since the 1500s. Maybe that's why there's a video game called Card Shark and a TV show called Card Sharks. A card shark might take a bite of your cards, but it's hard to play cards with fins. Best to avoid shark attacks and stick with card sharp.

If you've ever confused a card sharp with a card shark, you're not alone, as these examples show:

"When you're sitting at the table with card sharks, you can't trust anyone," he said. (Reuters)

From millionaire card sharks to home-game amateurs, players stake $10,000 each for a shot to win millions in poker's most popular variant. (Washington Times)

If you want to keep sounding sharp, keep that ace up your sleeve and remember that a tricky expert in card playing is a card sharp.