Tankie is a pejorative label generally applied to authoritarian communists, especially those who support acts of repression by such regimes or their allies. More specifically, the term has been applied to those who express support for one-party Marxist–Leninist socialist republics, whether contemporary or historical.
To me it is weird that every day on lemmy I see new posts complaining about all tankies... but I never actually see any of the content they are complaining about. And outside of lemmy, I never see or hear the word 'tankies' used at all. I've asked a couple of people I know in real life if they ever seen discussions about it in their parts of the internet, and none of them people I've asked have ever heard the word before.
So... like I said, I find it weird. It's like some kind of lemmy boogieman.