A tankie is a kind of person who like Imperialism and expansionism so long as it comes from something related to, as the other user said, Marxist-Lenism.

The origin of Marxist-Lenism is Russia, so they support Russia invading things, and committing warcrimes, and make all sorts of excuses for such acts.

Today they’re primarily in support of the Russian side in the Russo-Ukranian war, do not consider it an invasion of a sovereign nation, and consider it fully justified as an act to oppose “NATO expansion”.

Essentially, to a tankie, nothing “goes too far” if they can somehow connect it to a Marxist-Lenist agenda, and behave the same way about Putin as an right-wing American behaves about Trump.