SJW did indeed have a fairly concrete meaning when it first came into usage, but like with a lot of somewhat useful language, it got misappropriated by the right and rubbished so badly it barely resembles what it once was. Let me explain.
The idea was that a "social justice warrior" was someone who adopted progressive causes only as a guise to abuse others, that if they had a good cause, they could get away with being a bully.
The etymology of the term came from "keyboard warrior", someone who is aggressive and abusive online, behind the anonymity of the keyboard. That's what "social justice warrior" implies; aggressive and abusive. In the original meaning, it was the "warrior" part that was negative.
That's the implication of "SJW"; you're not sincere in your beliefs, you're only using them as a shield for your bullying. It's important to understand that while "SJW" is used as a catch-all, and applied broadly, it's not entirely meaningless. It still carries that implication.
In fact, that's often what a lot of right-wing catch-all terms imply; that you're hypocrites, you don't actually believe what you say you believe. In this case it's cover for being a bully, but it's the exact same thing with what "political correctness" implies.
The original meaning of political correctness was for example, a politician doing a thing that was politically beneficial to their career, but against their own values. Hence it was the "politically" correct thing to do, but morally incorrect.
Once again, the implication being that the person being "politically correct" is insincere in what they profess to believe, they're just saying whatever because it's expected. And yes, it's the same implication as "virtue signalling" etc. It's all the same core accusation.
This is the rhetoric the right constantly reinforces, so while it seems meaningless because it gets used so much in reference to just about everything, it means something to them: "You're all liars."