It originally was used to describe Leftists supporting authoritarianism while claiming to be leftist. It was used originally to describe Marxist-Leninist members of the Communist Great Party in Britain who supported the use of tanks by Soviets to quell a couple of uprisings in the late 50s and late 70s. It has since evolved as a term since then to include Leftists (or maybe not even leftists anymore, it's getting thrown around a lot now) who support similar actions in China and Russia. Usually they have an obsession with Mao and/or Stalin and China can do no wrong in their mind while claiming the western world is evil.

The term has an interesting history (to me as I'd describe myself as leftist, but I look at is as we all suck and can all improve) and Wikipedia has a decent article on it that I pulled most of this info from to double check my memory. Unfortunately they tend to be the most vocal part of leftist communities so I just tend to ignore any of them...

A tankie is a leftist who doesn't agree with mainstream geopolitical opinions or shows any interest in nuance

Tankies [1] don’t usually believe that Stalin or Mao “did nothing wrong,” although many do use that phrase for effect (this is the internet, remember). We believe that Stalin and Mao were committed socialists who, despite their mistakes, did much more for humanity than most of the bourgeois politicians who are typically put forward as role models (Washington? Jefferson? JFK? Jimmy Carter?), and that they haven’t been judged according to the same standard as those bourgeois politicians. People call this “whataboutism” [2], but the claim “Stalin was a monster” is implicitly a comparative claim meaning “Stalin was qualitatively different from and worse than e.g. Churchill,” and I think the opposite is the case. If people are going to make veiled comparisons, us tankies have the right to answer with open ones.

The reason we “defend authoritarian dictators” is because we want to defend the accomplishments of really existing socialism, and other people’s false or exaggerated beliefs about those “dictators” almost always get in the way — it’s not tankies but normies [4] who commit the synecdoche of reducing all of really existing socialism to Stalin and Mao. Those accomplishments include raising standards of living, achieving unprecedented income equality, massive gains in women’s rights and the position of women vis-a-vis men, defeating the Nazis, raising life expectancy, ending illiteracy, putting an end to periodic famines, inspiring and providing material aid to decolonizing movements (e.g. Vietnam, China, South Africa, Burkina Faso, Indonesia), which scared the West into conceding civil rights and the welfare state. These were greater strides in the direction of abolishing capitalism than any other society has ever made. These are the gains that are so important to insist on, against the CIA/Trotskyist/ultraleft consensus that the Soviet Union was basically an evil empire and Stalin a deranged butcher.

https://redsails.org/tankies/

AFAIK It's people who are pro-russia/china and vocal about it.

https://lemmy.world/comment/114872

https://en.wikipedia.org/wiki/Tankie