Here's how it likely developed:
* Grain milling: In the past, grain was ground into flour by using a millstone. This process involved a lot of movement and swirling of the grain as it was ground. This movement resembled the movement of people milling about aimlessly.
* Metaphorical extension: Over time, the phrase "milling around" evolved to describe any situation where people moved about aimlessly or without a clear purpose, just as the grain did in a mill.
While there isn't definitive historical evidence for this exact evolution, the connection to the milling process seems highly plausible.
The earliest recorded use of the phrase "mill about" (a close variation) is in the 1800s, suggesting the phrase was already in common use.