"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."
https://www.constitution.org/1-Constitution/cons/wellregu.htm#:~:text=The%20phrase%20%22well%2Dregulated%22%20was%20in%20common%20use%20long,calibrated%20correctly%2C%20functioning%20as%20expected.