Tankies are leftists that defend or deny the atrocities committed by authoritarian communist regimes like the Soviet Union or the CCP. It was first used to describe communists in Great Britain that defended the Soviet Union for using tanks to crush anti-communist revolutions.
Ironically history has proven that decision to have been the correct one and even the soft-left here in Britain today acknowledges that the people who were called “tankies” originally when that happened were all completely correct.
Edit: Downvoters literally defending against the stopping of what were literal actual fascist-led uprisings because they know absolutely nothing about european history.