the original origin of the term was a group british communists attacking anyone who supported the Soviet Union’s crushing of the hungarian uprising in 1956. it then morphed into a term used to attack anyone who supports the use of force and authority in general to suppress counter revolutionaries. it’s final degeneration is that it is now used to attack anyone to the left of an american democrat like facebones said.
here is a good article about it. To be clear: this is written from the perspective of a marxist leninist, who are normally the number one target of being called a “tankie”. Still, it is very short, and redsails is a really cool website that has the footnotes with citations pop up as you read long