"Bull feathers" is an interjection used to express disbelief. The concept is similar to "preposterous," which suggests that such an undeniable contradiction exists in the assertion being challenged that it cannot possibly be believed. In this case, the contradiction is clear: there is quite obviously no such thing as a bull with feathers.
Douchebag: You know, I got with this really hot chick last night and I'm totally not gay.
Critic: Bull feathers!
A person who wears a bullfeather is a person whose wife or girlfriend has cheated on him. The term comes from the "horns" on a cuckold -- the horn being the closest thing a bull has to a feather.
Just about any joke involving horns was considered hilariously funny through the 19th century, because everyone knew it meant the guy was a fool who was being cheated on -- the worst social disgrace for a man of that era.
John's wife is having an affair with Peter Quincy -- can't you see the bullfeathers on his head
Richard's girl sleeps with every guy in town. What a bullfeather he is!
Ruler of Lake Titicaca. Rumored to have a bunghole that gets very angry if it does not receive toilet paper. Cornholio the Great is often seen walking around with his shirt over his head and his hands in the air, chanting songs about his power, and his bunghole.
"I am Cornholio! You do not want to face the wrath of my bunghole, for I need TP!"
Butthead: Shut up, Beavis! (uh huh huh huh)
Beavis: Um, okay. (heh heh heh heh).