A different 'flash' is 'a flash in the pan' from the guns of a the 19th century.
It meant to splash, in the manner of falling water.
Cheap jewels and toys of the 17th century had many reflective surfaces.
The word dates back to the 14th century, when the first spelling was 'flasken'.
Here the gunpowder might ignite, but fail to properly explode to fire the bullet.
The idea that light might splash in this way is first recorded in 1548.
Therefore these were called 'flashy' meaning 'cheaply attractive', 'showy'.
