A thimble is a small pitted cup worn on the finger that protects it from being pricked or poked by a needle while sewing. The Old English word þȳmel, the ancestor of thimble, is derived from Old English þūma, the ancestor of the English word thumb. As the purpose of a thimble is to prevent discomfort while sewing by providing a barrier between fingertips and the blunt end of a needle, it is likely that the earliest thimbles were created closely following the invention of sewing.

Source: Wikipedia