The word energy had generally the same meaning when it was borrowed from Middle French energie in the 1590s, but the nineteenth century is when it really got established as a scientific concept as well as an everyday term. Energie is from Latin energia, which in turn is from Ancient Greek energeia, a word meaning "activity" (the verb form being energein, "to be in action"). The idea of that was developed by Aristotle in the fourth century BCE, and he described it by combining the prefix en-, which could mean "in", "at", or "within", and the root ergon, denoting work. Therefore, energy is "work within", which makes a lot of sense. Ergon derives from the Proto-Indo-European reconstruction werg, "to do", and usage of the word energy in literature over time has been decreasing since a peak in the 1980s.
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.