"Horsefeathers" is a term use to say that something is nonsense, often used as an exclamation after hearing something which one totally disagrees with. It is supposed to imply that an idea is as silly or as unbelievable as horses having feathers. The word is fairly new in origin; its first recorded use was in 1927, perhaps as a polite replacement for "horseshit". "Bull butter" is a similar euphemism, alluding to the absurdity as a bull producing butter, and at the same time avoiding the profanity of "BS".