considering there’s a few answers already out I’m assuming you didn’t want to scroll through all the comments.

short answer: a leftist that supports authoritarian regimes because they align with leftist beilifs.

longer answer: it comes from the soviet supression of the hungarian revolution. (it was bloody), where the british supporters of the soviets that supported the soviets were named tankies.

a modern tankie might deny the Uyghur genocide in china atm.