From urban dictionary:

Tankie by Zoldam

ℹ️ Description Slang for someone who supports militant communism, as opposed to the subversive, political, or economic warfare variants of communism. Generally, though not always, a Stalinist.

📌 Examples Only tankies would actually want to spread the revolution though military might.

Source

Or

Tankie by Whette Aeggue

ℹ️ Description 1 Communists who are anti-american imperialism to the extent that they defend cartoonishly bad people/countries as long as they claim to be socialist and challenged the interests of the U.S. or some other western nation. They can be found defending Joseph Stalin, Bashar al-Assad, and North Korea.

It began as an insult to members of the Communist Party of Great Britain (CPGB), who supported the Soviet Union's response to the Hungarian Revolution (1956) and the Prague Spring in Czechoslovakia (1968)

2 People who fetishize the aesthetics of 20th century socialist states and treat communism like a social group rather than a political ideology; not everyone who calls themself communist on Twitter is a tankie.

📌 Examples Tankie: "I try not to argue with people Lenin would have shot." T: "Come back after you've read theory, radlib." T: "Happy birthday, Stalin! 🎉🎊"

Source

Derogatory term for authoritarian leftists, particularly when the speaker is also a leftist.

It derives from the soviets sending in tanks to quell the Hungarian Uprising and the CCP sending in tanks into Tienanmen Square.