Definition of linetard
A "linetard" is an "online" (online) player who is "late" (delayed). Online + Delay = linetard. This is obviously an insult. You may see the expression pass in your online poker's chat room. Not all players on a table of poker have the same skills. Pros who have respect for beginners or less experienced players will call them "recreational players" or "recreational players". But when someone wants to insult a player, they will call him a linetard, and usually the linetard doesn't know he is linetard. There is an adage in the poker's that "80% of poker's think they are better than 80% of poker's".
Don't call anyone a linetard
If you're a pro player and you've just taken a serious badbeat by a player you call a "linetard", don't tell him, tell him "nice hand" or "well played" instead. Treating a player as a linetard could have several negative effects on you. For example, you could lose your chat privileges in your online poker-player room, you could encourage your opponent to study harder to the point where their knowledge is greater than yours, you could encourage them to leave the table, and so on. Other than breaking down, there is no advantage to treating a player of linetard.