It's Polish in origin, as ⟨bydło⟩ ['bɨd.wɔ]. In the language it refers directly to cattle, but metaphorically to cattle-like people, like a herd of people who don't think by themselves.
The way that you spelled it in your nickname is as Russian borrowed the word from Polish, ⟨быдло⟩ ['bɨd.ɫə]. In Russian I think that the meaning as "cattle-like people" is default.