What is the noun form for busy?

The noun related to the adjective 'busy' is business, formerly spelled/spelt busyness .

The earliest written record of 'busyness' was in 1849, in a work by Henry Thoreau.

In due course the idea of 'a busyness' being a good Word for a regular occupation or trade became separate and distinct from the idea of just being busy, which is what 'a busyness' really means. So the spelling for 'a business' meaning an entity in itself was gradually changed from a general busyness to a business.