Sharks, sharps, flats.

We played a lot of card games in my house when I was growing up, and if someone were to have a particularly good run my mother might call him a card shark. Interesting to note that was kind of a misnomer, except it wasn't. 

The term card shark is well-known, and was the name of a popular game show, but it doesn't make much sense. While the term shark is used for someone very driven at work (especially lawyers), we don't usually append the term shark to an activity to show someone is good at it. A good cook is not a kitchen shark; a great writer is not a word shark. But a pool shark is someone really good at pool, and for the same reason as the card shark -- it comes from cheating.  

We're gonna need a bigger pot.

In centuries past, sharks were not considered the magnificent beasts that the aquariums tell us they are now, but rather were considered parasites, ones that fed on others, as with loan sharks. So we might think that a card shark is either a mighty beast or a parasite that lives on smaller prey, but that may not be how the term originated. 

The word sharper as a noun likely came to the English language from the German schärfen, for sharpen, a way of calling someone a cheater, at least according to Grammarist. I suspect it may come from cheaters doctoring card decks by trimming or notching particular cards in a subtle way so they could tell what their opponents were holding. Oddly, I haven't seen that possible explanation online, but we know that deck doctoring is why new cards come in sealed boxes -- to avoid such tricks. 

Over time the card sharp, a kind of odd phrase, seems to have accidentally become card shark. But while the card sharp may be a cheat, a card shark is more often someone who's just really good at card games. (Different dictionaries, however, will define the terms differently.) It has been my experience in the real world that calling someone a card sharp is an accusation of cheating, but calling him a card shark is not. Whether the player is a card sharp or shark, though, he's not someone you want to go up against. Or at least, you'd best be a master at either method of play to go against him. 

Personally, that's why gambling has been the one vice that's had limited attraction for me -- there is no limit to the amount you can lose, and in a short time. What fun is that? I work too hard for my dough.