This term was first applied to the royalty of Spain during the Renaissance. The job of royalty, back then, was to loll around the palace, attempting never to lift a finger for any reason.

Because of their total lack of physical exertion, their blood was cool, lacking the oxygen induced by exercise. As a result, their veins actually showed through their skin, and so it appeared that their blood was pale blue.

—David Feldman, from Why Do Clocks Run Clockwise? and other Imponderables, 1988.