From a popular belief that the King could cure the disease by touching the sufferer.
king's evil (uncountable)
- (archaic) scrofula
1806, Robert Willan, On vaccine inoculation:Suppose the person the matter is taken from, has the king's evil, the pox, madness, or some other inveterate disease — I am sure the inoculator can give no reason why it should not convey one distemper as well as the other.