Someone who is skilful at playing or manipulating cards, or one who makes a living by cheating at cards.
'Card-sharp', which is sometimes spelled either 'card sharp' or 'cardsharp', might be thought by some to be a misspelling of 'card-shark'. The latter is the more commonly used of the two synonymous phrases, especially outside the UK, which is one of the few countries to prefer card-sharp. It is sometimes suggested that one term derived from the other. There's no clear evidence to support that view, although if it is the case then it must have gone from 'sharp' to 'shark' as 'card-sharp' appears to be the older term.
Both 'card-sharp' and 'card-shark' originated in the 19th century. There is a 1594 painting by the Italian artist Michelangelo Merisi (Caravaggio), that is called 'The Cardsharps'. Of course, Caravaggio didn't title his paintings in English and it isn't clear when it was given its Anglicized name - probably not until well into the 20th century.
Such tricksters were also known as broadsmen or spielers and 'card-sharping' was also called 'Greekery' - a derogatory term that probably wouldn't get past the political-correctness lobby these days.
The reason for thinking that 'card-sharp' and card-shark' may be independent coinages is the existence of the two much earlier words 'sharping' (swindling or cheating - circa 1692) and sharking' (cheating, stealing or sponging - circa 1608). These terms for deceitfulness have been adopted in to other phrases, for example, 'sharp practice' and 'loan shark'. Tricksters were called both 'sharps' and 'sharks' well before the 19th century, which makes the separate coinages entirely plausible.
Whatever the think about how and when the terms were coined there can be little doubt about where. Both 'card-sharp' and 'card-shark' appear in print in the USA many times before they are seen in publications elsewhere - a sure sign of country of origin. The first such devious card players were called 'card-sharpers' rather than 'card-sharps', although the dates of the earliest known citations of the two terms are close enough together to raise doubts as to which came first. 'Card-sharpers' was recorded by George Augustus Sala, in his Twice round the clock, or the hours of the day and night in London, 1859:
"German swindlers and card-sharpers."
As mentioned above, the earliest known citations of 'card-sharp' and 'card-shark' come from America. The first of these is in the New York Correspondence column of the Kansas newspaper Freedom’s Champion, from September 1859:
"Few of your men of the ‘Far West’ have any idea of the ups and downs of a stock speculator. It is true you may occasionally have the example of a card sharp who yesterday drove his tandem and only to-day is obliged to go afoot…"
'Card-shark' comes a few years later as in this example from Wisconsin newspaper The Daily Northwestern, October 1893:
"A few days ago Charles Petrie opened a gambling; house, which was promptly raided by the city police. Then Petrie got angry and swore out warrants for all the other keepers until every card shark in the city was taken in."
It seems over-generous to have two almost identical terms for the same thing and, in time, no doubt one - probably 'card-shark', will do to 'card-sharp' what grey squirrels have done to red squirrels. Until then, vive la différence.
See other phrases that were coined in the USA.