From a conversation with member Frances Heales; my comment about the Picts painting their bodies; her explaining the dyes having medicinal value and me doing more research:
Woad is a perennial called Isatis Tinctoria. Isatis comes from the Latin word Isazein and the Greek word Isadso and is linked to the ancient use of treating wounds. Tinctoria refers to its use in dye works for textiles. The Celtic people used woad indigo to make themselves look scary. The Romans called them Picti which means painted people. They colored their skin and pricked their skin and rubbed woad in to make tattoos. Now we know what the ancestors were up to.