réservoir

French réservoir derives from réserver, to keep back, itself from Latin reservāre, to hold in reserve. In seventeenth-century France, the word described any basin or tank that stored water for later distribution. It was practical, municipal, unpoetic. Cities needed réservoirs the way they needed roads.

English adopted the word in the late 1600s, keeping the French spelling and pronunciation. The early English reservoirs were engineering projects: the New River scheme that brought fresh water to London from Hertfordshire, completed in 1613, was among the first to use the concept at urban scale. The word named the structures that made cities possible.

By the nineteenth century, reservoir had expanded beyond water. An oil reservoir, a reservoir of knowledge, a reservoir of goodwill. The metaphor was irresistible: anything stored in quantity, held back for future use, waiting to be drawn upon. The word contained both abundance and restraint.

Medical science gave the word a darker sense in the twentieth century. A reservoir of infection, a disease reservoir. The same word that described a city's clean water supply now described the animal populations harboring viruses that could spill over into humans. Holding something back, it turns out, is not always benign.