Nice to have you here! This post is meant to poke fun at how "tankie" is used a lot online now.
It's usually used as a pejorative term against communists, but the people who use it tend to not really know what communism is or who it should apply to. You'll get funny situations like anarchists getting called tankies.
It harkens back to British communists a couple decades ago. People who supported the USSR's use of tanks to crush fascist protests in Czechoslovakia were called tankies. It's then devolved to applying to any communist that thinks a communist state should be able to use violence, and now it's become basically meaningless.
It's not a term we self identify with - just one that gets lobbed at us a lot.
Sorry if that was a lot! :).
*edit: A Nazi uprising in Hungary in 1956, per redsquid below