The term originates from controversy among UK Communist labor-unionists over support of the Soviet Union, in its violent occupation of dissenting Communist countries Hungary and Czechoslovakia — when Khrushchev sent tanks to suppress popular revolutions against Soviet control of their countries.

Notably, this was Communist-on-Communist violence: the revolutionary Hungarian and Czechoslovak regimes were still run by their Communist Parties. Thus "tankies" were, originally, Communist labor-unionists who endorse or tolerate violent suppression of other Communists to secure the power of the Soviet Union.

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

More recently it's been used to refer to supporters of authoritarianism in China, which is associated with tanks by way of the 1989 Tienanmen Square massacre.

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

It's a term mostly used to distinguish left-authoritarians from left-anti-authoritarians, particularly in the context of the cold war West. 'Tankies' notably tend to defend their left-ideology by talking up major authoritarian or totalitarian regimes (like Stalin's or Mao's) that also called themselves Socialist (despite such claims being problematic in doctrinaire terms). Originally used to describe the self-proclaimed communist/Stalin apologist, the modern tankie supports pretty much any authoritarian regime that opposes the West

Derisive term for communist authoritarian who support left wing ideology and also state violence to maintain their power.

Common themes include whitewashing Mao and Stalin history, and dismissing current state sanctioned violence against minorities in “communist” countries.