Insult in MOBAs, particularly League of Legends, that is used to refer to low-level players who treat the current metagame as gospel despite the fact that it's far more relevant for pros than it is for your average schmuck in solo queue. When you rage at someone for doing something sensibly unconventional regardless of how well it's working while asking everyone to report them, you're being a metasheep.
Notice how I said "sensibly unconventional", however. Sensibly unconventional is sending a champ who doesn't normally go in a particular lane into that lane to said lane because they counter the enemy pick for that lane. It is NOT sending a champ who is woefully unsuited for a role to do it anyways just because it's unconventional. You're not breaking the meta by doing the latter, you're just being a little prick. People doing the latter sometimes use "metasheep" as an epithet for the teammates who are quite rightfully bitching them out for it, but they're still in the wrong.
"omg noob swain cant top gg i afk"
"Shut the fuck up, retard metasheep."
"0/5/0? What the fuck, Vayne? How could you think that going mid vs Panth was a good idea?"
"lol stfu fag metasheep"