The story
A modern translation of Latin, virtus dormitiva, coined by Molière in The Imaginary Invalid (1673). In the play, he lampoons a group of physicians providing an explanation in macaronic Latin of the sleep-inducing properties of opium as stemming from its "virtus dormitiva". The currency of this phrase as a critique of scientific claims is due to Gregory Bateson (1976, Steps to an Ecology of Mind p. 5), as is the translation of virtus as 'principle'.
Etymology from Wiktionary, CC BY-SA 4.0.