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mid-15c., "the weighting of a fishing line," verbal noun from plumb (v.). Specific meaning "water and drainage pipes" is recorded by 1875, American English.
THE apparatus by which the water from a reservoir is carried about over a building and delivered at points convenient for use, is called by the general name of plumbing. The word "plumbing" means lead-work; and it is used to signify this water apparatus of a house because the pipes of which it largely consists are usually made of lead. [Edward Abbott, "Long Look House: A Book for Boys and Girls," Boston, 1877]Alternative plumbery also is mid-15c. Slang meaning "a person's reproductive organs" attested by 1975.
"lead hung on a string to show the vertical line," early 14c., from Old French *plombe, plomee "sounding lead," and directly from Late Latin *plumba, originally plural of Latin plumbum "lead (the metal), lead ball; pipe; pencil," a word of unknown origin, related to Greek molybdos "lead" (dialectal bolimos) and perhaps from an extinct Mediterranean language, perhaps Iberian.
"perpendicular, vertical," mid-15c., from plumb (n.). The notion of "exact measurement" led to extended sense of "completely, downright" (1748), sometimes spelled plump, plum, or plunk.
Completely; entirely; stone: What he said was plumb silly
[1748+; fr notions of exact extent and precision associated with the plumb bob or sailor's plumb line (for measuring depth of water), ultimately fr Latin plumbum, ''lead'']
(Unix) Term used for shell code, so called because of the prevalence of "pipelines" that feed the output of one program to the input of another. Under Unix, user utilities can often be implemented or at least prototyped by a suitable collection of pipelines and temporary file grinding encapsulated in a shell script. This is much less effort than writing C every time, and the capability is considered one of Unix's major winning features. A few other operating systems such as IBM's VM/CMS support similar facilities.
The tee utility is specifically designed for plumbing.