It comes from the 1950's when a section of the 'left' who believed the Soviet Union was inherently good found themselves defending the Soviet invasion and forced annexation of Hungary. The soviets lead the invasion with Tanks, hence 'Tankie'.
This support happened due to the simplistic belief that communism was inherently good, capitalism was bad and so the actions of a Communist country (like the Soviet Union) could never be bad if taken against a capitalist country is inherently good.
With the effective collapse of communist countries 'tankies' has switched to the people on the left that view western society as evil and any country which attempts to destroy or subvert western countries.
This typically manifests in a 'tankie' defending any action/supporting any view taken by China, Iran, Russia or North Korea and typically and typically amplifying anti USA rhetoric.
For example a Tankie will argue the Russian invasion and attempted genocide of Ukraine is justified and the USA is 'evil' for helping Ukraine defend itself.