The way I’ve heard it used in online gaming is not just that the person is bitter, but that they’re spewing complaints about some perceived injustice. It’s term making fun of and dismissing the people who constantly complain when they’re not winning.
I find it amusing that, aside from slang that specifically references a piece of technology or modern-only phenomenon, someone always seems to find an example of slang being used by ‘kids today’ before WW2.