Typically it refers to leftists who strongly defend/advocate for authoritarian statist approaches to socialism/communism.

A cute water cistern.

While someone's political beliefs are highly multi-dimensional, there are two axes that are commonly used to define where someone sits:

• Economy - Left is favouring social responsibility for people receiving economic support (supporting people to meet their basic needs is everyone's collective responsibility), while right is favouring individual responsibility (meeting your basic needs is your responsibility, and if you die because you can't, even if it is due to something outside of your control, tough luck).
• Social liberties - Social Libertarian is favouring individual decisions on anything not related to the economy / rights of others, while Social Authoritarianism supports government restrictions on social liberties.

Since there are independent axes, there are four quadrants:

• Socially liberal, Economic left - e.g. Left Communism, Social Democrat, most Green parties, etc...
• Socially authoritarian, Economic left - e.g. Stalin, Mao. Tankie is a slang term for people in this quadrant.
• Socially liberal, Economic right - Sometimes called libertarian. Some people with this belief set call themselves Liberal in some countries.
• Socially authoritarian, Economic right - e.g. Trump. Sometimes called conservatives.

That said, some people use tankie as cover for supporting socially authoritarian, economic right but formerly economic left countries(e.g. people who support Putin, who is not economically left in any sense).

OP, what have you unleashed.

A size of tank, smaller than normal tank, but bigger than tankette.

It's what centrists call anyone to the left of netanyahu.

Like most words it can mean different things depending on context. I’ll do my best to cover a few without spoiling it with my own opinions.

The most common usage is as a blanket pejorative aimed at anyone who identifies as leftist but also openly endorses authoritarian means or ends.

There are also those who embrace the term and they are also not all the same. There are Marxist-Leninists who believe the only path to a stateless egalitarian society is through a revolutionary vanguard party. There are also those who argue that egalitarian society can only be achieved and maintained through benevolent authoritarianism.

In any case, the term carries an implication of authoritarianism and/or revolutionary violence, hence “tanks.”

A slur mainly on the internet against those leftist (usually Marxists-leninist) who oppose western interventions, sanctions, coups and wars against countries and governments labeled as "authoritarian".

originally used by UK communists party trotskyist wing in support of Hungarian -56 crushed uprising against those who opposed it, calling them tankies. Vaguely same as "stalinist", but it pretty much has lost that meaning in modern use.

A lot of these comments reminds me of people posting on conservative chats asking what a socialist is. About the same amount of mouth foaming.

I would like to remind people about vaccinations against rabies

It's essentially a pejorative for "communist." I recommend the Prolewiki article on "Tankies," as well as Nia Frome's essay "Tankies."

For those that want an introduction to Marxism-Leninism, I made an introductory Marxist-Leninist reading list, check it out!