Someone who plays a multiplayer video game and refuses to use any weapons/characters/skills that aren't considered "meta."
Although someone who uses the meta can be someone who simply enjoys using the most reliable tactics available, "Metaslave" is used to describe someone who sees anyone who doesn't use the meta as an idiot or noob and may even berate those who refuse to comply with their demands.
Originated from League of Legends to describe someone who uses an overpowered champion in order to climb up the competitive ranks.
"Hey, why did Todd get banned from the community server?"
"He kept yelling at this dude for using a machine gun instead of an SMG. He was promptly called a Metaslave then perma-banned."
Someone who only picks or uses top-tier or "in the meta" characters in a game. This can include hero picks in mobas, deck lists in card games, or a specific setup for a character class in an RPG.
Someone who ONLY follows the meta(Most Effective Tactic Available)and doesn't try anything else apart from the ever-shifting meta regardless if its fun or not and thus are like a slave to the meta.