"Tankie" is a pejorative term for doctrinaire and militant communists. The term dates back to the 1950s, and the Soviet invasion of Hungary. While many academic Marxists or other leftists condemned the brutality of the invasion, many of the most adamant communists in the United Kingdom and The United States, among other places, thought the decision to use tanks to crush the revolt was justified. The name "tankies" derives from this.
With the fall of communism and the passing of the 20th century, the term "tankie" should have fallen onto the ash heap of history. But then something happened. Well, a few things happened. One of those things was twitter, and one of those things was 2016. Certain sectors of the less thoughtful parts of the internet, including twitter, came to be a base for communists of various stripes to spout their dogma. Thus, other activists and leftists brought the word "tankie" out of retirement. The meaning has somewhat mutated, as well, in part because of the tragic Syrian Civil War. The term "tankie", originally used to describe communists, has also been extended to apply to anyone who defends dictators or violent regimes, simply because they are opponents of American foreign policy.
It is a sad and confusing year when we have to revive a term such as this, and a sign that the freedom of communication that the internet has promised us has failed us in many ways. Also, a sign of how many things have flipped in the past year: "Social Justice Warriors", a bête noir for many conservatives, in general are one of the major detractors of "tankies", while many "conservatives" are now showing sympathy to dictators that were once their primary enemies. Of course that is a simplification of sorts, but we need to simplify 2016 if we are to understand it even a little.