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.