It likely originated in the early 20th century and became popularized through vaudeville and radio. The exact origin is unknown, but there are a few theories:
* From horsehair stuffing: Horsehair was once commonly used to stuff furniture and mattresses. The phrase may have emerged as a way to express disbelief or ridicule, implying something was as absurd as stuffing a bed with horse feathers.
* From "horseplay": "Horseplay" is rough, boisterous playfulness. The expression "horse feathers" could have evolved as a way to dismiss such playful behavior as silly or ridiculous.
* From a theatrical gag: Some believe it originated as a catchphrase used in a vaudeville act, possibly involving a character who wore a ridiculous hat made of feathers.
Regardless of its exact origin, "horse feathers" became a popular way to express disbelief or to dismiss something as absurd. It's often used humorously, but can also be used in a more serious context to express strong disagreement.