Tankie was first used for that kind of communist supporter who kept singing Russia's praises/defending Russia even when Russia was sent 5000 tanks to crush a popular uprising in Czechoslovakia (the "Prague Spring") on August 20, 1968. Some people just couldn't accept that a communist country could do something bad, so defended the action.

Nowadays, it's used to refer to those that are strongly supportive of Russia, completely ignoring the awful things they do. Often these days there's a lot of anti American bent to it. Like, anything anti America and American "imperialism" must be good - even blatant and awful Russian Imperialism.

These days they calmly explain how Ukraine just needs to come to the table and discuss peace (ignoring that Ukraine wouldn't exist if they did so) and blame America for the war in Ukraine for... well... they're America. The people who want war, or are causing the war, are those giving Ukraine weapons - not the country that is literally invading it.