When introduced from France in the thirteenth century, the word “pannier” denoted nothing more than a basket, originally for the carrying of bread, whence its name, from the Latin panarius, bread basket.
But it had also come to mean a fish basket or, if of larger size, a basket for the carrying of provisions of any kind.
When someone in later centuries conceived the brilliant notion of balancing two of these larger baskets across the back of a donkey, these too became panniers.
It was but a step, then, to transfer the name to the basket-like frames which, at the demand of fashion, women affix beneath skirts to extend the size of the hips.