This is very interesting to me because, as someone who spends a lot of time on Tumblr (shut up, I know), which skews heavily toward the young-politically active-AFAB demographic, I see the word “trash” used overwhelmingly as a slightly humorous self-descriptor, generally combined with something you’re totally into.
e.g.:
“But then, I’m Reylo trash, so I’m gonna see everything through that lens” (Reylo - the belief that Rey and Kylo Ren will be/should be in a relationship)
“Yeah, me and a bunch of other Hamiltrash took over the karaoke and did the whole soundtrack.” (Hamiltrash - big Hamilton fans)
etc
As LMM once commented “When the kids really like a thing, they call themselves the trash of the thing.”
It can also be used in a more self-depricating than self-descriptive sense, e.g. “And of course I didn’t get the living room clean on time, because I’m trash that way.”
But the point is, it’s mostly used as a descriptor of oneself. To use it against someone - to call them “Reylo trash” - is a lot more insulting and implies that not only is the person you’re talking about trash, but also everyone else who likes Reylo. (Ship wars are a Big Deal.)
I think it comes at least partly from the word “eurotrash”, which I’m not sure of the definition of - it seems to be an East Coast word.
Anyway, I believe it comes from this demographic originally, as a lot of terms do. I believe I read an article a while back that said that most slang terms that are widely adopted originate in one of two communities - either the Black community, or young (white) women/teenage girls.
Ah! Here’s a different article on the young-women’s side of it. Move over Shakespeare, teen girls are the real language disruptors