Origins
The word 'energy' is unusual among scientific terms in having been coined by a philosopher for a metaphysical concept and only later adopted by physicists for a measurable quantity. It enters English in 1599 from Late Latin 'energīa,' from Greek 'enérgeia' (ἐνέργεια), a word that Aristotle developed into one of the central concepts of his philosophy. The Greek word is built from 'energós' (ἐνεργός, active, at work), composed of 'en-' (ἐν, in, at) and 'érgon' (ἔργον, work, deed, action), the latter from PIE *werǵ- (to do, to work).
The PIE root *werǵ- has a remarkable dual lineage in English. Through Greek 'érgon,' it produced 'energy,' 'synergy' (working together), 'allergy' (other-work, an abnormal reaction), 'liturgy' (public work, originally a civic duty performed for the state, later a religious service), 'metallurgy' (working with metal), 'surgery' (hand-work, from 'cheir' + 'érgon'), 'organ' (a working instrument), and 'George' (earth-worker, farmer). Through Germanic, the very same PIE root produced Old English 'weorc,' modern English 'work,' and German 'Werk.' Energy and work are thus etymological siblings — a fact that physics would independently confirm when it defined energy as the capacity to do work.
Aristotle's use of 'enérgeia' was philosophically precise and distinct from modern usage. In his Metaphysics and Nicomachean Ethics, 'enérgeia' meant 'actuality' — the state of being fully realized, of functioning according to one's nature. He contrasted it with 'dýnamis' (δύναμις, potentiality, capacity, power). An acorn has the 'dýnamis' to become an oak tree; the mature oak is the 'enérgeia' of that potential. A harpist playing the harp is in a state of 'enérgeia'; the skill lying dormant is 'dýnamis.' For Aristotle, 'enérgeia' was the more fundamental concept — actuality is prior to potentiality, because potentiality is defined by reference to the actuality it aims at.
Development
The word entered English with a general sense of 'force,' 'vigor,' or 'intensity of expression.' In rhetoric, 'energy' meant forcefulness of speech — an energetic oration was one delivered with power and conviction. This rhetorical sense was common through the seventeenth and eighteenth centuries.
The scientific transformation came in 1807, when the British polymath Thomas Young proposed 'energy' as a technical term in physics, specifically for what we now call kinetic energy — the energy of motion, calculated as half the mass times the velocity squared. Young's choice was apt in ways he may not have fully appreciated: kinetic energy is indeed 'dýnamis' become 'enérgeia' — potential made actual, capacity translated into activity. Over the following decades, the concept expanded to encompass potential energy (Rankine, 1853), thermal energy, electromagnetic energy, and eventually Einstein's mass-energy equivalence (E = mc², 1905).
The Greek root 'érgon' also hides in several English words where it is less obvious. 'Allergy,' coined by the Austrian pediatrician Clemens von Pirquet in 1906, combines 'állos' (other) + 'érgon' (work) — an 'other-working,' the body responding abnormally. 'Surgery' comes from Greek 'cheirourgía' (hand-work), from 'cheír' (hand) + 'érgon.' 'Liturgy' comes from Greek 'leitourgía,' from 'lêitos' (public, of the people) + 'érgon' — originally a public work or civic service that wealthy Athenians were required to perform, such as funding a warship or a theatrical production, later transferred to the public worship service of the Christian church.
Greek Origins
The modern proliferation of 'energy' into everyday discourse — energy drinks, energy healing, 'good energy,' 'bad energy' — represents a return, in some respects, to the word's pre-scientific vagueness. But the underlying Greek metaphor persists in all uses: energy is work-in-action, potential made real, the force that transforms capacity into deed.