In poker lingo, the term “fish” is used to describe an extremely unskilled player who tends to play a loose game, but remains passive regardless of the situation at the table, or makes aggressive moves when he shouldn’t. The term was popularized well before internet poker became mainstream, so it’s equally prevalent in land based card rooms.
“Hooks are for fish. You’ve got to flop ’em, fold ’em, or fornicate ’em.” -Johnny Moss referring to a pair of Jacks
The term probably originated from the act of fishing – a clueless fish is easy to catch with proper bait, becoming food for the man with a rod and an appropriate skillset. It’s also worth pointing out “fishiness” is fairly relative – as one of the characters of John Dahl’s “Rounders” put it, “If you can’t spot the sucker [fish] within the first half hour at the table, then you are the sucker.”
The only thing that remains constant is the fact that the more skilled players “eat” the stacks that belong to the weaker ones, creating a poker food chain of sorts. Consequently, the strongest players are often called sharks, as there’s no sea creature that would consider them an easy prey.