oscillate

Latin oscillare meant to swing, specifically the motion of something hanging and moving back and forth. The noun at its root was oscillum: a small mask of Bacchus, carved in wood or terracotta, that Roman farmers hung in their vineyards on cords. The mask swung in the breeze, turning on its cord, and farmers believed it brought good luck to whichever vine the face happened to face when the wind dropped. Virgil mentions the custom in his Georgics, written around 29 BCE.

The Latin oscillum was almost certainly a diminutive of os, the word for mouth or face. A little face: that is what hung in the vineyard. By the classical period the word had already generalized from these specific ritual objects to any hanging, swinging thing, and the verb oscillare had extended to cover any back-and-forth motion. Roman writers used it for pendulums, for balanced scales, and for the rocking of ships in harbor.

The English verb oscillate entered the language in the early 18th century, arriving through natural philosophy as scientists needed precise vocabulary for pendulums and vibrating strings. Galileo had established the isochronism of the pendulum around 1602, demonstrating that a pendulum's period is independent of its amplitude. A century later, the mathematical analysis of oscillatory motion was a central concern at British and Continental universities, and oscillate filled the need for a single verb covering any repetitive back-and-forth movement.

By the 19th century oscillate had extended beyond physics into rhetoric and politics. Speakers who shifted between positions oscillated; markets oscillated between fear and greed; political opinion oscillated around a center. Today physicists speak of oscillating circuits and oscillating fields, while novelists use the same word to describe a character who cannot choose between two lives. The Bacchic masks spinning in a Roman vineyard have become entirely invisible inside it.