The term was originally coined when the USSR sent Russian T54 tanks into Budapest, Hungary on the 4th of November 1956 to suppress a worker uprising. Factories had been taken over nationally by workers councils, in a demonstration of worker self-organization that was at odds with the Soviet's imperialist rule. The Soviet troops eventually suppressed the uprising and restored their rule. Then the USSR sent the tanks in to invade Czechoslovakia in 1968, and Afghanistan in 1979. A decade later, in 1989, tanks were similarly used by another state-capitalist regime to crush student dissidents in Tiananmen Square in China.
Anarchists use the word "tankie" to describe any supporter of authoritarian regimes that claim to be socialist. "Red fascist" is another popular term used in this context. The exception is Hitler's "national socialists", who are simply referred to as fascists. Like Lenin and Stalin, Hitler initially represented himself as a socialist; realizing that appropriating socialism would be useful to gain popular support. Of course, his genocidal actions had nothing to do with establishing socialism, and his so-called "national socialist" ideology was just another form of collectivist-capitalism.