early 15c., quint-essence, in ancient philosophy and medieval alchemy, "a pure essence latent in all things, and the substance of which the heavenly bodies are composed," literally "fifth essence," from Old French quinte essence (14c.) and directly from Medieval Latin quinta essentia, from Latin quinta, fem. of quintus "fifth" (from PIE root *penkwe- "five") + essentia "being, essence," abstract noun formed (to translate Greek ousia "being, essence") from essent-, present-participle stem of esse "to be" (from PIE root *es- "to be").
The Latin term is a loan-translation of Greek pempte ousia, the "ether" that was added by Aristotle (perhaps following the Pythagoreans) to the four known elements (water, earth, fire, air) and said to permeate all things. It was naturally bright, incorruptible, and endowed with circular motion. Its extraction was one of the chief goals of alchemy.
The transferred or figurative sense of "purest essence" (of a situation, character, etc.), "an extract from anything containing in a small quantity its virtues or most essential part" is by 1560s.