Definition of livetard
A "livetard" is a "live" player (in a casino or hall) who is "retarded" (delayed). Live + Delay = Livetard. It's obviously an insult. You may hear the expression at the casino you go to, or in your card room. Not all players at a poker table have the same skills. Pros who have respect for beginners or less experienced players will call them "recreational players". But when you want to insult a player, you'll call him a "livetard". Usually, the livetard doesn't know he's a livetard. Livetards are also known as "old-timers", i.e. older players who have been around since the very beginning and who are being outclassed by the younger newcomers.
Don't call anyone a livetard
If you're a pro player and you've just taken a serious badbeat from a player you describe as a "livetard", don't tell him or her, instead say "nice hand" or "well played". Calling a player a "livetard" could have several negative effects on you. For example, you could lose your right to play at your casino, you could get kicked out, you could encourage your opponent to study harder to the point where his knowledge exceeds yours, you could encourage him to leave the table, etc. Apart from venting, there's no advantage to calling a player a livetard.