Over time, the meaning of "indolent" shifted from simply meaning "not feeling pain" to describing a state of laziness, inactivity, and aversion to work or exertion. This shift in meaning likely occurred because people who are physically inactive tend to experience less pain than those who are active, and thus, "indolent" became associated with a lack of physical activity and a preference for ease.
Interestingly, the chemical indole is named after the same Latin root "indolens," although it's not clear why. It's possible that the chemical's name was inspired by its odorless nature, which could be seen as similar to the "painless" sense of "indolens."