'A well-regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.'
Originally Posted by SGray
I think we need a bit more emphasis on the bolded portion of the amendment
We can begin to deduce what well-regulated meant from Alexander Hamilton's words in Federalist Paper No. 29:
The project of disciplining all the militia of the United States is as futile as it would be injurious if it were capable of being carried into execution. A tolerable expertness in military movements is a business that requires time and practice. It is not a day, nor a week nor even a month, that will suffice for the attainment of it. To oblige the great body of the yeomanry and of the other classes of the citizens to be under arms for the purpose of going through military exercises and evolutions, as often as might be necessary to acquire the degree of perfection which would entitle them to the character of a well regulated militia, would be a real grievance to the people and a serious public inconvenience and loss.
--- The Federalist Papers, No. 29.
Hamilton indicates a well-regulated militia is a state of preparedness obtained after rigorous and persistent training. Note the use of 'disciplining' which indicates discipline could be synonymous with well-trained.
There are other cites to had for the definition of the phrase. Clearly not what Sgray intended.