"Tankie" was a term used to describe those that supported Khrushchev sending tanks to hungaria during that whole thing. Now, it's been swallowed by western "leftists" and spit out to mean, in summary, "i can't imagine anyone being more left than me, so they must be secret fake leftists that are actually fascists and golly gee i'm so smart." comrade brain_in_a_box said it very well that it's just the lib/soccdem/anarchist version of "woke:" just a random buzzword to refer to anything they don't like.