Also, if you have noticed sniffing bums really isn't pleasent. So People will also take it in as a hurtful response. Just like fuckyou, or bitchplease. IT's all the same. Bumsniffe is just the nicest form to put it in :)
You can also say it randomly. Try it, It's fun.
Person 1 : "Hey ______, you look like a douche today"
Person 2 : "Shut up Bumsniffer"
Random Person 1 : "....Bumsniffer"