The term originates from Soviet and aligned regimes sending in tanks to brutally crush protests and rebellions. E.g. The Hungarian Revolution, The Prague Spring Uprising, Tiananmen Square, etc. Some communists were disgusted at their fellows for cheering on said oppression (“Send in the tanks!”) and started calling them Tankies.
Tankies fellate oppressive regimes and dictators. They’re the smooth-brained “communists” that live in a binary world where anything “their side” does is good and anything the west does is “evil”. They’ll claim any criticism of historically “communist” countries like China and Russia is a CIA talking point … because they’re idiots.