plumbum

Latin plumbum meant "lead"—the soft, heavy, easily melted metal that Romans used for pipes, roofing, weights, and sling bullets. Its chemical symbol, Pb, comes from this Latin name. A plumb line was a string with a lead weight (a plumb bob) tied to the end. Gravity pulled the weight straight down. The string marked true vertical. It was the simplest and most accurate leveling tool in the ancient world.

Roman plumbers—plumbarii—worked with lead pipes. The entire Roman water system depended on lead plumbing. The English words "plumber" and "plumbing" come directly from plumbum. Whether Roman lead pipes actually poisoned Romans is debated—hard water deposits may have coated the interior surfaces, reducing lead contact. But the association between plumbum and water delivery is literal: plumbers are, etymologically, lead-workers.

"Plumb" entered English as an adjective meaning perfectly vertical, and by extension, perfectly correct or complete. "Plumb crazy" means completely crazy. "Plumb forgot" means completely forgot. The lead weight that defined vertical became a word for absolute precision. To plumb the depths was to measure how deep water was by lowering a lead weight on a marked line—a technique used in navigation until the 20th century.

In 2014, the city of Flint, Michigan, switched its water supply to the Flint River. The corrosive water dissolved lead from the city's aging plumbing. Lead contamination affected roughly 100,000 residents. The crisis was a direct consequence of the Roman engineering decision to use plumbum for pipes—a choice still embedded in the infrastructure of American cities and in the English word for the trade that maintains it.