Dirtbag to me always just meant adopt the same sort of behaviors that fash adjacent people find funny and effective, the whole bullying, demeaning behavior against people you don’t like ie don’t self censor or pay too much attention to verbalized ableism, racism, misogyny, homophobia etc.
But you can’t really be a ‘dirtbag’ and a leftist. Someone can write it out better than I can but, like, they want to evoke images of ‘real communism’, what the hard nosed Soviet Union supposedly represented. But lost in all their bravado was strength and power. Not verbal bullying and that’s all that dirtbag is, calling someone an ugly hard r and essentially running in the same spaces as most right wingers.