If you're looking for a relatively serious answer:

In the 1960s the Soviet Union sent military aid to their ally Hungary to squash a fascist attempt at overthrowing their socialist government.

This was presented in the western media at the time that the evil soviets were attacking the "free speech" of these (fascist) protestors. So those who approved of the decision were labelled as "tankies" as a kind of derogatory term.

The term was usually used by "leftcoms" or people who claim to be socialist or leftist, but explicitly exclude every socialist nation from "socialism" for not being pure enough.

Until the rise of the term on the internet, where the term saw a resurgence, where the original meaning has almost been entirely lost, and usually just means "anyone who is actually left wing and opposes capitalism and imperialism."

It's usually just used because "Commie" sounds too much like boomer redscare propaganda. But it has the exact same meaning as a boomer screaming "Commie" at someone who says that maybe free healthcare is a good idea. It's a thought terminating cliche, a way of dismissing someone's argument without directly engaging it and considering their position.