It is the Scottish name for the flower "Sweet William."
OR
The Scots sometimes refer to the noxious ragwort, not Dianthus barbatus, as "Stinking Billy" in memory of the infamous Duke
It goes back to the Battle of Culloden in 1746, in which the forces of Prince William, Duke of Cumberland massacred Scotsmen.
https://en.wikipedia.org/wiki/Dianthus_barbatus