A more authroritarian leftist, as oppossed to an anarchist.

Tankie refers to people who support authoritarian "communist" regimes and often deny or justify the atrocities they commit. Stalinists mostly, but also supporters of the authoritarian aspects of China and even North Korea

The internet's superpower is to drain words of meaning.

Initially the word Tankie was a very specific, very directed insult. It was a pejorative for like. Y'know, Soviet-Union simps. "Leftists" that spent a lot of time memorizing Soviet Union WW2 trivia and adorning themselves in Soviet ephemera, much like how certain nazi teens do. The accusation was very direct and very clear: "You do not care about the workers' cause. You just like the Soviet aesthetic and have a very weird thing for ww2". Like a Wehraboo except they'll call you 'Comrade' instead of a racial slur.

It then, by simple extension, became a pejorative for Marxist-leninists. Cuz of course. Most MLs are Soviet Union simps, later they transitioned to being China simps and would you look at that, China also offers a lot of military ephemera they can be, y'know, weird about. Of course if you lack a bit of context you can then use it for any vaguely authoritarian 'leftist' and you won't even be all that wrong, it's close enough.

Eventually though? It sorta leaked (thanks a lot, Adam Something)

... And now there's a lot of like, straight up Neoliberals using the term 'Tankie' as an insult in the same way their conservative grandparents would have used the term 'Pinko'. It has entirely exited the purview of leftist discourse.