Eventually it drifted from it's original meaning to instead mean "Marxist-Leninist, or an adjacent Marxist ideology (anything from Trots to MLM or MTW, but mostly ML) which is an ideology I hate due to American propaganda," and I think it was basically used that way for a long time; then more recently it's become a term for "Anyone who doesn't unequivocally condemn designated enemies of the US state." This more recent drift means an anarchist or even a social democrat who doesn't support Azov Battalion, the IOF or Syrian "moderate rebels" can be labeled a tankie, making it essentially a meaningless term.

The nice thing is that if someone calls someone else a tankie you can basically immediately know that the first person is some disgusting globe emoji ghoul.

EDIT: Of course, you sound like you already know this, but for anyone who doesn't, the original meaning was "ML who didn't support the fascist counterrevolution in Hungary."