I understand that the usage actually started much later, but the root of the idea is the split in European communist groups after the Hungarian invasion of '56 between those who essentially bought the Soviet justification and those who didn't.
I wouldn't say its simply those who support the Soviet Union but rather those who fully buy into Soviet propaganda to justify their actions, for example that Czechoslovakia was being taken over by Western sponsored fascists necessitating a Warsaw Pact invasion rather than the need to end the threat that a democratic socialist government would pose to the rest of the bloc's control over their own people.
Nowadays it tends to refer to leftists willing to back pretty much any country that acts in opposition to the US/NATO despite not really having any real socialist credentials themselves (like Iran or Russia) or being a total basket case (like North Korea).







