I found this rather showy (indeed gallant) flower on a roadside near where I live.
It is viper's bugloss.
The bugloss part comes from its rough stem and leaves - bugloss means ox-tongue from Greek bous + glossa. The viper component is because the plant was once thought to be sovereign against snakebite.
Culpeper's Herbal of 1653 says
"It is an especial remedy against the biting of the Viper, and all other venomous beasts, or serpents."
Why did people believe this? Had it been found, by accidental trial and error that the plant worked against snakebite and so its use for this purpose became part of local tradition? Does in fact save people from death by snakebite?
No, the reason seems to come from the Doctrine of Signatures. According to this concept, which goes back to antiquity, plants with medicinal uses bore signs that indicated what they were good for.
So for example the leaves of lungwort were thought to look like diseased lung tissue, showing that it would be good for lung ailments.
This notion seems odd to modern thought, but it's based on a belief in providence: these plants had been put there by a creator, or perhaps just by nature, but for a purpose - to help humankind - and so the plants should be labelled - as the products in a supermarket aisle should be. For me the hardest bit is imagining that the world was created solely for us. That seems so unlikely and species-centered. But we are using modern thought and it's fairly easy to at least empathize with a different mindset.
So following the DOS, the stems of viper's bugloss are speckled, something considered characteristic of snakes (think of the Sherlock Holmes' tale The Speckled Band) and so the herb is marked as a snakebite remedy.
Culpeper offers several other uses for viper's bugloss, for example as a cheery pick-me-up.
"There is a syrup made hereof very effectual for the comforting the heart, and expelling sadness and melancholy."
I could do with some of that but I'll wait for proper testing!