What does plumb mean?
To plumb something is to measure its depth, traditionally with a weight on a line, and the word has developed a figurative meaning of examining a subject or situation in depth. “Plumb the depths,” for example, suggests looking beneath the surface of difficult or complex matters. The adjective “plumb” describes something exactly vertical or properly aligned, as in “out of plumb,” meaning not straight or true. In literary or regional usage, “plumb” can also mean completely, as in “plumb exhausted.” The word is related to the English noun “plumb,” a weight used to find a perpendicular, and comes ultimately from Latin plumbum, the word for lead. This metal accounts for the association with plumbing and with the ancient practice of using lead pipes.
To measure the depth of something, especially water, or to investigate a subject or situation thoroughly.
"The team spent the afternoon plumbing the river and analyzing its flow."
The literal use is common in surveying, engineering, and diving contexts; the figurative use is widely established in formal writing.
Exactly vertical, properly aligned, or completely so.
"The carpenter checked that the post was plumb before securing it."
In the sense of exactly vertical, “plumb” is often used in the established phrase “out of plumb.”
A small weight, traditionally made of lead, attached to a line and used to establish a vertical or perpendicular reference.
"He lowered the plumb into the well to measure its depth."
The noun is now relatively uncommon outside technical, historical, and specialized contexts.
Completely, thoroughly, or to an exhaustive degree.
"After the long climb, the climbers were plumb tired."
This use is chiefly literary, dialectal, or archaic.
The plural is rare and generally used when referring to multiple plumb weights or instruments.
"The surveyors checked their plumbs before beginning the measurement."
The word comes from Latin plumbum, meaning “lead,” the metal once used in pipes and still associated with plumbers.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does plumb come from?
The English word comes through Old French from Latin plumbum, meaning “lead.” Latin plumbum also gave English the words “plumber” and “plumbing,” reflecting the historical use of lead in water pipes. The English sense of measuring depth developed from the use of a lead weight on a line, while the meaning “exactly vertical” comes from establishing a true perpendicular.
Related word forms
How plumb is actually used
The verb is common in technical and formal contexts, while “plumb the depths” and similar expressions are metaphorical. “Plumb” as an adverb is chiefly literary, regional, or archaic, and is often used in phrases such as “plumb tired.” As an adjective, “plumb” can mean vertical, exactly aligned, or completely so, depending on context.
Easily confused with plumb
Plumb means to measure depth or investigate something, whereas plume can mean a feather, a decorative plume, or to rise or billow.
Plumb is the English word for the action of measuring depth and for vertical or exact; plumbum is the Latin word for lead.