Short answer: folk who are so pro-Communist that they think the Tiananmen Square massacre was a good thing. The massacre in which the CCP bulldozed protestors with tanks.
Hence, "tankie"
Edit: apparently I'm wrong on the origin of the term, and it predates Tiananmen Square. See the reply below.
I will add that given the current projection of power currently, China is a far more powerful authoritarian communiststate than Russia, so most tankie propaganda is China-centric these days.
Wrong spectrum. Both Republicans and Tankies are authoritarian, which is why they feel similar (especially because current republicans are barely even conservative and have mostly gone all in on the authoritarian part).
It's a word liberals use to dismiss anyone to the left of them. in the same way Maga dipshits say 'woke' about liberals. Just an amorphous slur that means let's stop talking about issues that make us question the dominant narrative.
[Aaaand here come the downvotes which will prove my point]