Where does the term “tankie” come from?

So, I was a total bystander in a very heated discussion recently. It seemed like one of the participants was a total war-hawk regarding this Ukraine situation. The guy on the other side seemed to be a total apologist for Putin.

The war-hawk called the other person a "tankie." Both seemed to know what the term meant. And it sure seemed to be taken as an insult.

Where does the term "tankie" come from? What does it mean?