Leftists sometimes use it as an insult because liberals are still capitalists or because liberals in the U.S. have an ineffective "reach across the aisle, " "civility politics" reputation. I've mostly heard it from tankies (who are not leftists, IMO) or from leftist using it in more of a tounge-in-cheek manner though.
Essentially just boils down to: “You care about other people you evil scumbag. Not like me, I’m a patriot, I’ve got mine, screw everyone else.”
I've learned that liberal has a very different meaning if it comes from tankies, communists or non-americans than it does if if it comes from a Republican, and my knowledge of world politics is honestly too limited to fully understand why that is and if it's for different reasons for each of the above groups
But I agree with the person you're replying to, everyone I've encountered who uses "liberals" as an insult has not been a terribly pleasant person to interact with.
Or I never see communists here talk about fascists and the far right the same way they talk about liberals or anyone else on or near the left who doesn't quite meet their purity test. Almost as if they prefer the company of nazis.
It means "brigading", specifically "brigading by shitpost", basically "bullying". They use the term themselves for their own behaviour on communities outside of their own instance here on lemmy, and it is very typical of their authoritarian beliefs.
You say a thing that pisses them off on any random community.
They link and @you in their "dunk" community and try to bait you into replying there.
Or, they just use that post as a rally to get people to come to the thread you were on.
They then hammer you with "well america is terrible too" and genocide denial and just general shittiness. Essentially trying to one up you any way they can, then congratulating themselves on "dunking" you.
What's perplexing is how the use of terminology (everyone is librul) is straight from the capitalist newspeak dictionary. Why the insistence to follow and strengthen capitalist dogma but be against capitalist hegemony in name