The first time I saw this antiquated form was in Steeleye Span's interpretation of The Elf Knight ballad, but I tend to see it now and then and I don't quite know how it is used.
Lady Isabel sits a-sewing
Fine flowers in the valley
When she heard the elf-knight's horn a-blowing
As the rose is blown
What is the name, role, history (how it came to be, and how it died) of that prefix? Does it affect the gerund in any way, modifying its meaning or such, or is it just "the way it was spoken back then"? Is this related to the indefinite article
a put in front of normal nouns, treating the gerund as any noun, or is it something completely different?