I was sure that it had something to do with the distance that could be ridden by horses, but according to our friend Wikipedia, it comes from an Old English word (*þriding ) which descends from an Old Norse word (þrithjungr) for ⅓ of [a county].

Source: https://en.wikipedia.org/wiki/Riding_(country_subdivision)

Thanks for asking this. I thought to look it up a couple of times today to satisfy my own curiosity, but didn’t quite get there.

thanks for this too. I was also curious.

Yorkshire in England used to have the West, East and South (or North?) Ridings.