“Tankie” is not synonymous with “socialist”. It was coined to describe a type of person who will side with the war crimes of anybody who waves a red flag regardless of whether they’ve fallen into statist counterrevolution.

“Nazi” is short for “national socialist”, because they fraudulently wore the name of socialism whilst doing horrific counterrevolutionary repression. Tankies are the left’s name for another type of person who does almost the same thing, although with more token adherence to the aesthetics and ideology of supposedly leftist thinkers and movements.

Some people call them “red fash” although there is a specific subgroup of tankies who I think that label fits better, the nazbols.