No flies on you

What is the meaning and origin of the saying 'No flies on you'?

It's a 19th-century slang phrase implying that someone is so brisk and active that no flies have a chance to settle on them. (VSD)

I'd add only that in the 19th century more people lived a rural life, and even if they lived in the city, they saw a lot of horses. Horses and cows ans other ungulates soon become covered with flies when they are standing still, as you are reminded when they flick their tails.
SS