a little mass or weight of lead, or the like, attached to a line, and used by builders, etc., to indicate a vertical direction; a plummet; a plumb bob. see plumb line, below.-perpendicular; vertical; conforming the direction of a line attached to a plumb; as, the wall is plumb.-in a plumb direction; perpendicularly.-to adjust by a plumb line; to cause to be perpendicular; as, to plumb a building or a wall.-to sound with a plumb or plummet, as the depth of water; hence, to examine by test; to ascertain the depth, quality, dimension, etc.; to sound; to fathom; to test.-to seal with lead; as, to plumb a drainpipe.-to supply, as a building, with a system of plumbing.