Originally it was a label Trotskyites used to criticize Stalinists (aka ML's). A bit of inter-communist sectarianism. The specific line was something like "they support where ever the soviet tanks roll". Specifically it came from the suppression of the Hungarian revolts. But it more broadly meant having uncritical support for the USSR and everything it does.

But now we're 30+ years after the fall of the USSR, so its definition has spiraled into any westerner who opposes western imperialism. For example: an anarchist might use it to criticize an ML outright cheering Russian victories in Ukraine. In turn, a liberal will use it to criticize the same anarchist for opposing NATO.