‘A well regulated militia.’
FWIW, I think it refers to the fact that when it was concocted, a standing,professional army of sufficient strength did not exist; it was necessary to buttress with the various militias, which were private citizens who gathered together on a consistent basis, for training with their own weaponry...
It’s been pointed out that in the 18th century “well regulated” meant `trained & drilled in the use of small arms’, not `closely regulated and tightly controlled’.
Militia men who met on the village green to practice close order drill & who could deliver clean volley fire, were said to be well regulated.
Militia men who met on the green mainly to quaff ale, not so much.