The term "tankie" was originally used by dissident Marxist–Leninists to describe members of the Communist Party of Great Britain (CPGB) who followed the party line of the Communist Party of the Soviet Union (CPSU). Specifically, it was used to distinguish party members who spoke out in defence of the Soviet use of tanks to suppress the Hungarian Revolution of 1956 and the 1968 Prague Spring, or who more broadly adhered to pro-Soviet positions.
I've never understood why there is any confusion over the word "tankie." It applies to the pro-cop left. If a leftist believes that it's necessary for cops to beat minorities and dissidents into submission for their society to function, they're tankies. If they approach leftism in a way that does not involve state violence against civilians to enforce those ideas, they're not tankies. To me there isn't a lot of gray area.