means The system of pipes, fixtures, and fittings that carries water in and waste out of a building — or the trade of installing and maintaining it.

from From the Latin 'plumbum,' meaning lead — the soft, heavy metal the Romans bent into their water pipes. That same root gives us the chemical symbol Pb and the word 'plumb' (a lead weight on a string for finding true vertical). A 'plumber' was once literally a worker in lead; the meaning drifted to anyone who handles the pipes, long after lead itself fell out of favor.