Tankie has been a thing for over half a century. The Soviets used tanks(from other countries) to invade and crush antisoviet revolutions/protests in Hungary and Czechoslovakia.
This was considered shocking for some communists and other leftists while some communists supported those actions. Those who supported this "intervention" were called tankies, in order to differentiate them from the non tank invasion supporting communists.
In Greece(my country), which was in the "West", this event caused the split of the greek communist party(KKE) into 2 separate parties, KKE(Moscow controlled tankies) and KKE "interior"(weak ass soy communists who cant stomach the use of tanks in order to create a better tomorrow).
KKE "interior" kinda eventually became SYRIZA which surprisingly won the elections in 2015(breaking the 2 party status quo). Then the EU decided that the greek people decided poorly and punished them. So when the greek people decided correctly(elected a right wing government), the EU rewarded them. Carrots and sticks make the world go around. Thats how you make a majority leftist country, into a right wing one.
PS Greek debt is as high as it has ever been but Greece issued negative interest bonds. Tell me again how is the economy supposed to work? Noone talks about the greek debt anymore, it's a non issue, Greece has been accepted into the EU's infinite money glitch scheme.
Well i and other people that i know were using the term for at least 10 years. I even had a friend break up partly because the other person was a tankie. Maybe the term was less common in other countries.
Also tankie definition hasnt changed nor has been appropriated as a general slur. It has always meant communist who supports authoritarian policies.