Someone who engages in the Meta-game of online games, so deeply is his/her addiction to it, they live it each day, each hour and let it define their lives by it.
"Eve online/MMO" is a great example of this behavior as is any game with a thriving/ active "Meta" Community there are always those who thrive in this virtual world of spying, doxxing and data-mining peoples personal information for a in-game advantage.
"Holy fuck this twat is one huge Metatard."
"Ignore them, they are huge Metatards"
"Oh no..... he has gone full MetaTard"