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