The Onion

To my understanding this means historically a western communist who supported the USSR's interventions (with tanks, hence the name) in Eastern European countries (Hungary, Czechoslovakia, maybe also East Germany) when the Warsaw Pact was a thing. More broadly someone who thinks the Soviet Union was a good regime.

Tankies are people who claim to be communists, but who exclusively support authoritarians and their oppressive regimes, past and present, because these authoritarians call themselves communists (think believing Nazis were socialists because it was in their party name), even though their actions have nothing at all to do with communism, and fascists co-opting leftist language to gain power being a well documented phenomenon.

It's pretty easy to tell really: if a project is/was stateless, classless, and moneyless - congratulations, it's almost certainly communism, and those who support it are communists (or possibly one of several varieties of anarchist, like myself).

However if one or any of those conditions aren't met, and/or there is a single dictator or a small group of cronies holding on to power claiming they've been met, but the reality is clearly different - congratulations, you basically have run of the mill fascism on your hands and those who support it are tankies (who, as I pointed out in OP, almost always speak up and in defence of a "dear leader" but never in defence of the people themselves, because all they care about is the regime, not the people living under it).

In the case of the fediverse, there are a couple of dedicated instances almost exclusively occupied by tankies, who have made it their goal to essentially troll and brigade anyone who doesn't agree with their, frankly bizarre, views (calling it "dialectics" to make themselves feel better). They're generally easy to avoid though, by blocking individuals, or their entire instance.

From what I've been told, it originally was communist countries that use tanks against their people (like Tienanmen Square). It's getting muddled by the countries that use tanks against their people, so I don't know what it has become.