The Draper’s Dictionary (1882) blandly says, “The name corduroy is of French origin, where it was originally corde du roi, the king’s cord.”
But, alas, no such name has ever been used in France.
The French name for the material is velours a cotes, “ribbed velvet.”
First mention of the fabric was in the 1790’s, but no hint accompanied that or later mention as to the reason for the name.
It has the semblance of clever advertising.
Material disguised as “the king’s cord” would undoubtedly interest a prospective buyer.