The current usage refers to people who back dictatorships that call themselves socialist.

Historically, the definition was slightly tighter. A few years after the death of Stalin, the USSR brutally crushed a revolt in Hungary with tanks. The UK's communist party was okay with this. This led to a lot of people leaving the party and calling the people who remained "tankies".

Over time, the term became more broad. A lot of the original users of the term supported Stalin, and would be considered tankies themselves by the modern definition.

It is also sometimes misused. Some people think that dictatorships are the only form that socialism can take. As a consequence, these people call all socialists tankies, including anarchists and demsocs.

"Tankie" is a term referring to authoritarian communists, due to the tendency of authoritarian communist states to use tanks in crushing protesters - such as the Soviet Union crushing Hungarian workers in 1956, or China rolling tanks into Tienanmen Square against student protesters in 1989. "Tankie" is also used for apologists for these regimes.

Tankie is sometimes misused as "Someone to my left I don't like".

Try posting a meme critical of Russia's invasion of Ukraine to lemmy.ml's meme community. If it isn't immediately taken down for "rule 1: uncivil", you will catch some people pasting long entries defending Russia in the comments with varying degrees of civility. These will be the ones we call tankies.