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.
Why not both? They make fun of the technical knowledge of a userbase, you respond by insulting them directly for the non-technical ideology held by someone twice removed (while, coincidentally, providing an example for their point).