Nobody seems to know the derivation of the term "horsefeathers." The most likely explanation is it began as a sanitized variant of "horse hooey", used to express the view that something is unlikely, about as improbable as that pigs might fly... or that horses should have feathers. By adorning her Pony with large, multi-colored feathers, this former art teacher from Texas has given the term a totally new form of expression. Resin.