Has “..Well-Regulated Militia...” ever been legally defined?
Yes, clearly defined in the Federalist papers.
It had a definite meaning when it was written - it meant “practiced”.
Indeed it has. If you look at the state constitutions in effect at the time our constitution was written many of them defined it as “every able bodied man between ages of 18 and 50.” Militia service was very popular, in part because in many states, NY in particular, it allowed an exemption to the property qualification to be able to vote. Maybe they didn’t feel the need to define it because everyone was assumed to know what it meant.