A saying used by gamers to describe how they were killed/cheated death. A play on the word frag tag, only the lag on the server is the reason you died/ avoided a serious can of ass whoopery.
Gamer 1: I hate this game, I just emptied a whole mag into that guy and all he does is look at me and I'm dead. Fucking lag tag!
Gamer 2: Ha Haa, lag tag mother fucker. You can't catch me! boom!!! yo dead lol