wear the purple

Tyrian purple was made from the glands of murex sea snails, thousands of them per garment, in a process so foul-smelling that the dyeworks were kept outside the city walls. The result was a deep red-purple that did not fade, and it cost more by weight than gold.

Rome turned the expense into law. A senator's toga carried a purple stripe; a general celebrating a triumph wore a toga entirely of purple and gold; and under the emperors, the full purple became effectively an imperial monopoly, with severe penalties for anyone else who wore it. In Byzantium the association went further — an emperor's child born during the reign was porphyrogenitus, born in the purple.

English kept the metaphor after the dye itself became cheap. To wear the purple, or to be raised to the purple, is to hold the highest office; to be born in the purple is to be born into that family.