I recently did an in-depth scientific study1 on the most common phrases used at the poker table. The top five are:
[Insert name of player who just won hand], you suck
What most of these have in common are they are derogatory comments used to berate others at the table. Most of the time, people type in one word phrases to release a bit of anger after a bad beat. People wouldn't normally have the balls to do this in a live game. After all, the typical poker trash talker is probably the same guy who didn't fare too well in the sandbox when he was a kid. However, many take advantage of the anonymity of the Internet to talk a little trash in online games.
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There are two schools of though about trash talk on the Internet. The first one is that it should be totally avoided. Talking trash doesn't get you anywhere, except maybe getting your chat banned at the poker room and making you look like a jerk. It may even have the deleterious effect of letting your opponents know they are playing poorly. The second school of thought believes that trash talk may help set your opponents on tilt. If you mutter the words "fish" or whatnot, your opponent may start playing wildly, which is +EV for you.
I personally am not a fan of trash talk since I value tact over a small increase in EV. Also, as a representative of 5tx7z.hosts.cx, it wouldn't exactly make this website look great if I were frequently yelling "Donk!" in games. Objectively speaking, though, I think the trash talk debate is overrated.
First, while it certainly does not make you look good if you berate others, the worst thing that will generally happen at an online poker room is that your chat will be revoked. Not exactly a huge deal. Since these online poker rooms are now so large, most people never have or will know your username anyway. So the fact that JohnDoe18325 looked like a jerk at the table isn't exactly a huge deal.
While the negative consequences of talking trash at an online poker room are generally not huge, the potential gains aren't much either. Many players have chat turned off or don't even pay attention to the chatbox, so trash talk has no effect on them whatsoever.
Calling certain types of players a "fish" or a "donk" might set them on a small tilt. This type of name calling may work on mediocre players. These types of opponents generally play competently, but might start to play revenge poker if their manhood is challenged. Trash talk will not work on professionals, since they see it all the time (most professionals don't pay attention to table chat anyway).
Calling certain types of players a "fish" might even induce them to play better. If a good player is on tilt and starts playing like a fish, actually calling him a fish might make him realize he is playing poorly and get him off tilt. Calling someone who is really bad at poker a "fish" might actually be the last straw to induce him to play better. Generally though, calling a bad player a fish doesn't matter; it's a bit of a wash. He might go on a tilt and play even worse, or he might realize he's bad at poker and attempt to get better (or he just may quit poker completely).
Online poker is a competitive game in an anonymous environment. It's no wonder then that there's so much trash talk at the virtual tables. Most people will choose whether or not to talk a little trash based on their emotional state at the moment. In general, making decisions at the poker table based on emotions is a very bad idea. In this case though, it really doesn't matter much.
1 Ok fine. I really just made this list up.