Per the origins of the term, a tankie is a communist that supported the Soviets wuelling the Hungarian 1956 uprising. It was an insult concocted by British Trotskyists, who also consider themselves communists.

The modern use of the term is just a liberal sentiment leveled against anyone that doesn't fall neatly in line with US Empire's vilification campaigns. If you dare to say that Russia has material motivations that are a counter to those of the US rather than being a kingdom run by a madman that just loves killing, you are a tankie. If you don't want Ukraine used as a proxy for the US to hurt Russia, regardless of how many Ukrainians die, you are a tankie. If you treat the PRC as country filled with normal people living normal lives rather than the dystopian nightmare it's falsely depicted as, uou are a tankie. If you know anything at all about Dengism, you are a tankie.

Really, the liberal position on both countries is premised on orientalism and it is never a surprise when the criticisms inevitably turn into vague tropes. And when this laziness is called out, well, it's time to deploy a tactical tankie reference. I definitely don't care about being insulted, these situations are really just a way for the other person to give themselves an excuse to stop thinking or engaging.