Definition of livetard
A "livetard" is a "live" player (in a casino or room) who is "late". Live + Delay = Livetard. This is obviously an insult. You may hear the expression in the casino you go to or in your gaming 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. Generally, the livetard doesn't know he's a livetard. Livetards are also referred to as 'old-timers', i.e. the oldest players who have been around since the 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 them so, 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 cause your opponent to study harder to the point where his knowledge exceeds yours, you could cause him to leave the table, etc. Other than venting, there is no benefit to calling a player a livetard.