The word Tankie originates from 1950s British Communist circles. Specifically, it was used by British Communists to derisively describe their comrades who supported the 1956 invasion of Hungary by the Soviet Union.

Images of the Soviet invasion featured a lot of tanks, hence, "Tankie".

After that died down, the term didn't come back into use really, until the 2010s, when leftists on the internet started using it in a tongue-in-cheek sort of way. It was fun to bring back a stupid sounding, incredibly niche, British slang word.

At some point the word breached containment and started to be used by liberals, in a very cavilier sort of way. I've seen people use Tankie to describe anyone from Marxist-Leninists, to Marxists generally, to Leftists generally, weird right-wingers who converted to Russian Orthodoxy, pro-Palestine activists, mods of Lemmy instances someone doesn't like.

Shit, I've seen literal Anarchist get called Tankies.

Basically, it's a meaningless nothing word now, that's a bit like your boomer grandpa who still thinks it's the Red Scare, calling Joe Biden a Commie Pinko.

So don't worry about it too much.