It’s a meme where someone issues a threat against someone else, then veils it against moderation & reporting by appending “in Minecraft.” e.g. a user responds to your post with “I’m gonna burn your house down”, which can be construed as a real-world douchebaggery. But if they say “I’m gonna burn your house down in Minecraft”, there’s plausible deniability because it’s infers online douchebaggery where real physical harm is unlikely.

I think it started with people saying “I would like to (do something very illegal) to (some person) in Minecraft.” Thinking that saying “in Minecraft” would shield them from any repercussions because they only wanted to do it in a simulated environment. Eventually some people would just shorten it to things like “…in Minecraft” and leave the obvious part unsaid.

Everyone always says “terrible thing” and then adds “in Minecraft” on the end to absolve themselves of all liability.