horsefeathers
Nonsense; indicates disbelief.
Dismissing the rumors as "horsefeathers", he advised everybody to disregard them.
Long hair on lower legs of a draft horse (e.g., Clydesdale), especially the rear legs.
Feathering strips.
The exterior of the court is entirely of sheathing and wood shingles applied over "horsefeathers."
Where it comes from
The "nonsense" sense, a euphemism for horseshit, is horse + feathers, a nonexistent item as only birds have feathers. Compare hen’s teeth. The "horse hair" sense notes the similarity of that hair to bird feathers. Regarding the roofing sense, compare feathering strip and horsefeathers.
Adapted from the Wiktionary entry horsefeathers (CC BY-SA 4.0).