And I always thought it meant "trained" and practiced to the point of competence and effectiveness for the purpose of civilian and personal defense; as opposed to a trained standing army.
The following are taken from the Oxford English Dictionary, and bracket in time the writing of the 2nd amendment:
1709: "If a liberal Education has formed in us well-regulated Appetites and worthy Inclinations."
1714: "The practice of all well-regulated courts of justice in the world."
1812: "The equation of time ... is the adjustment of the difference of time as shown by a well-regulated clock and a true sun dial."
1848: "A remissness for which I am sure every well-regulated person will blame the Mayor."
1862: "It appeared to her well-regulated mind, like a clandestine proceeding."
1894: "The newspaper, a never wanting adjunct to every well-regulated American embryo city."
The phrase "well-regulated" was in common use long before 1789, and remained so for a century thereafter. It referred to the property of something being in proper working order. Something that was well-regulated was calibrated correctly, functioning as expected. Establishing government oversight of the people's arms was not only not the intent in using the phrase in the 2nd amendment, it was precisely to render the government powerless to do so that the founders wrote it.