“Tankie” historically refers to british communist who supported the military intervention against the hungarian counter revolution by the socialist world in 1956. The term was coined by british communists who did not support said intervention.

Now it refers to anyone opposed to nuclear war, WW3 or capable of nuance. Rule of thumb: If someone uses “tankie” unironically, you can safely ignore what this person has to say about anything. It’s the liberal “woke”, “SJW” and new “commie”, same thought terminating bullshit.