I have no clue where the word comes from, but my understanding is that it’s a hyper-authoritarian communist. Think Stalin or Mao Zedong. The kind of person who’s not afraid to start a genocide or a famine if it means the people dying of starvation are capitalists. You’ll usually see them defending or denying the crimes of the people I listed earlier plus others. Pol Pot is a good example.

I too have been called a tankie on here which is very strange considering I’m a bit less of a leftist than the majority on here.

Whatever meaning it had originally is gone and it’s basically just a way to discredit anyone vaguely on the left you don’t like.