ETYM AS. stapul, stapol, stapel, a step, a prop, post, table, from stapan to step, go, raise.
1. A short U-shaped wire nail for securing cables.
2. A short length of U-shaped wire used to fasten papers together.
In medieval Europe, a riverside town where merchants had to offer their wares for sale before proceeding to their destination, a practice that constituted a form of toll; such towns were particularly common on the Rhine.
In English usage, it referred to a town appointed as the exclusive market for a particular commodity, especially wool. The wool staple was established by the English crown in Calais 1353. This form of monopoly trading was abandoned 1617.
Necessary foods or commodities.

To secure or fasten with a staple

1. Used, needed, or enjoyed constantly usually by many individuals
2. Produced regularly or in large quantities
3. Principal, chief