The copy editor is in.
I'm presenting occasional posts on the use of English,
not to be pedantic but just for the fun of language.
This is one that people like to fight me on, but it's cardsharp. I don't doubt that enough people use cardshark now that it will become just as acceptable sooner or later. (Language is ever evolving, and it's a very democratic process.)
Sharp and shark as negative nouns actually probably both stem from the same root, sharker. Certainly the idea of a metaphorical shark being one who preys on others (witness loan shark and lawyer jokes) is well established. But sharp is more defined as an expert and/or a cheat, both of which relate more narrowly to card games.