Liberals think it means someone who lies about wanting socialism and instead just wants to kill people. They think it refers to a proponent of a society where all decisions are made by a single crackpot dictator. They think they have a correct view on how to create and institute socialism, which would involve things like voting in elections, organizing pickets, and very slowly seizing power through mass appeal. As a corollary to this, liberals believe this type of socialism has already been achieved in places like Sweden and Denmark. Depending on how much they've read socialist theory they may also point to Catalonia or the Zapatistas, but they'll usually be referring to completely imaginary mangled versions of these events that don't reflect history.

What tankie means in practice though is something more like "race traitor." Or "nation traitor." Liberals will say they hate the west's actions and what it stands for, but what they really mean is they hate everywhere else more. Any designated enemies of western countries are permanent enemies and outside of liberal values. People who call themselves socialists will still use this dichotomy. Tankie means someone who expresses sympathy for countries that have been designated as enemies to the west. You'll notice that Xi Jinping himself isn't called a tankie. Stalin's not a tankie. Figures like that get called authoritarians or dictators. Tankie is someone in the west who has broken convention and gone to the bad guys. That's all it means at this point.

I really believe there's been a decades long project to usurp socialist rhetoric by liberals, whether they realize it or not. The existence of tankies threatens their reputation, because it draws socialist terminology back into scary communism. Liberals have spent decades trying to position themselves as the true, good leftists. That only works if they can paint socialist movements worldwide as liars and their own internal socialists as traitors