okay, so I was on pinterest the other day and I saw a fanart of Bill possessed Dipper and the description was BIPPER, like in all caps someone was really excited about that fanart. I thought that Bipper was the ship name for Bill and Dipper and got kinda grossed out, were the shippers thinking it was like romantic that Bill was in Dipper's body? like that's kinda weird. But then I saw it on a few more Bill possessed Dipper fanarts (some of which were really creepy btw) and I decided that MOST OF THE TIME (there are some occasions where Bipper is used to describe a nerdy guy kissing a dream demon) Bipper refers to a Bill possessed Dipper while Billdip always means (a nerdy guy kissing a dream demon) I might be wrong but yeah that's just what I think.
soooooooooooooo
Bipper
Billdip
^^^^^^^^^NOOOOOOO
Hope this helps!