The Collaborative International Dictionary of English v.0.48:

Plumb \Plumb\ (pl[u^]m), n. [F. plomb, L. plumbum lead, a leaden
   ball or bullet; cf. Gr. mo`lybos, mo`libos, mo`lybdos. 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.
   [1913 Webster]

   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.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Plumb \Plumb\, a.
   Perpendicular; vertical; conforming the direction of a line
   attached to a plumb; as, the wall is plumb.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Plumb \Plumb\, adv.
   In a plumb direction; perpendicularly. "Plumb down he falls."
   --Milton.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Plumb \Plumb\, v. t. [imp. & p. p. Plumbed (pl[u^]md); p. pr.
   & vb. n. Plumbing (pl[u^]m"[i^]ng).]
   1. To adjust by a plumb line; to cause to be perpendicular;
      as, to plumb a building or a wall.
      [1913 Webster]

   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.
      [1913 Webster]

            He did not attempt to plumb his intellect. --Ld.
                                                  Lytton.
      [1913 Webster]

   3. To seal with lead; as, to plumb a drainpipe.
      [1913 Webster]

   4. To supply, as a building, with a system of plumbing.
      [1913 Webster]

WordNet (r) 3.0 (2006):

plumb
    adv 1: completely; used as intensifiers; "clean forgot the
           appointment"; "I'm plumb (or plum) tuckered out" [syn:
           clean, plumb, plum]
    2: conforming to the direction of a plumb line
    3: exactly; "fell plumb in the middle of the puddle" [syn:
       plumb, plum]
    adj 1: exactly vertical; "the tower of Pisa is far out of plumb"
    n 1: the metal bob of a plumb line [syn: plumb bob, plumb,
         plummet]
    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