Source : Webster's Revised Unabridged Dictionary (1913)

Plumb \Plumb\, n. [F. plomb, L. plumbum lead, a leaden ball or
   bullet; cf. Gr. ?, ?, ?. Cf. {Plummet}, {Plunge}.]
   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.

   {Plumb bob}. See {Bob}, 4.

   {Plumb joint}, in sheet-metal work, a lap joint, fastened by
      solder.

   {Plumb level}. See under {Level}.

   {Plumb line}.
   (a) The cord by which a plumb bob is suspended; a plummet.
   (b) A line directed to the center of gravity of the earth.

   {Plumb rule}, a narrow board with a plumb line, used by
      builders and carpenters.

Plumb \Plumb\, a.
   Perpendicular; vertical; conforming the direction of a line
   attached to a plumb; as, the wall is plumb.

Plumb \Plumb\, adv.
   In a plumb direction; perpendicularly. ``Plumb down he
   falls.'' --Milton.

Plumb \Plumb\, v. t. [imp. & p. p. {Plumbed}; p. pr. & vb. n.
   {Plumbing}.]
   1. To adjust by a plumb line; to cause to be perpendicular;
      as, to plumb a building or a wall.

   2. 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.

            He did not attempt to plumb his intellect. --Ld.
                                                  Lytton.

   3. To seal with lead; as, to plumb a drainpipe.

   4. To supply, as a building, with a system of plumbing.

Source : WordNet®

plumb
     adj : exactly vertical; "the tower of Pisa is far out of plumb"

plumb
     n : the metal bob of a plumb line [syn: {plumb bob}, {plummet}]

plumb
     adv 1: completely; used as intensifiers; "clean forgot the
            appointment"; "I'm plumb (or plum) tuckered out" [syn:
             {clean}, {plum}]
     2: conforming to the direction of a plumb line
     3: exactly; "fell plumb in the middle of the puddle" [syn: {plum}]

plumb
     v 1: measure the depth of something
     2: weight with lead
     3: examine thoroughly and in great depth
     4: adjust with a plumb line so as to make vertical