You know how people call Voldemort "you-know-who" or "he who must not be named"? It's the same energy, but for HN.

It's a website with a very marked orange header and some orange styling. Since there's very little else in the way of appearance (coming from me, that's a compliment!), the orange becomes quite prominent. Calling it "the orange site" seems very reasonable.

It's a "nickname" that people who don't like HN use.