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

Moby Thesaurus II by Grady Ward, 1.0:

288 Moby Thesaurus words for "plumb":
   T square, absolute, absolutely, accurately, all-embracing,
   all-encompassing, all-out, all-pervading, answer, aplomb, appraise,
   appreciate, assay, assess, at right angles, azimuth circle, bang,
   bar, barricade, batten, batten down, bob, bolt, born, bottom,
   broad-based, button, button up, calculate, calibrate, caliper,
   cast the lead, check a parameter, choke, choke off, clap, clean,
   clear, clear up, close, close up, come alive, completely,
   comprehensive, compute, congenital, constrict, consummate, contain,
   contract, cover, crack, dead, debug, decipher, decode, deep-dyed,
   definitely, delve, delve into, descry, dial, dig into, directly,
   discern, disentangle, divide, divine, do, dope, dope out,
   downright, dyed-in-the-wool, egregious, entirely, estimate,
   evaluate, even, exactly, exhaustive, explain, explore, expressly,
   faithfully, fasten, fathom, figure out, find out, find the answer,
   find the solution, flush, fold, fold up, full, fully, gauge, get,
   get right, go into, graduate, guess, guess right, have it, hit it,
   in all respects, in every respect, indagate, intensive, interpret,
   investigate, ipsissimis verbis, irretrievably, irrevocably, just,
   kerplunk, key, latch, lead, literally, literatim, lock, lock out,
   lock up, look into, make a sounding, make out, measure, mensurate,
   mete, meter, normal, occlude, omnibus, omnipresent, open the lock,
   orthodiagonal, orthogonal, out-and-out, outright, pace, padlock,
   peer into, penetrate, perceive, perfect, perfectly, perpendicular,
   perpendicularly, pervasive, pierce, plain, plop, plumb bob,
   plumb line, plumb rule, plumb the depths, plumb-line, plummet,
   plump, plunging, plunk, point-blank, poke into, positively,
   precipitous, precisely, prize, probe, pry into, psych, psych out,
   pure, puzzle out, quantify, quantize, quite, radical, rate, ravel,
   ravel out, regular, resolve, riddle, right, right angle,
   right-angle, right-angled, right-angular, rigidly, rigorously,
   sandbag, seal, seal off, seal up, search into, secure, see,
   see daylight, see into, see the light, see through, set,
   set square, sheer, sheerly, shut, shut the door, shut up, sift,
   sinker, size, size up, slam, slap, sling the lead, smack,
   smack-dab, snap, solve, sort out, sound, span, spang, spot, square,
   squarely, squeeze shut, stark, steep, step, straight, straight-up,
   straight-up-and-down, strangle, strictly, survey, sweeping,
   take a reading, take soundings, thorough, thoroughgoing,
   thoroughly, through-and-through, to the letter, total, triangulate,
   try square, ubiquitous, unconditional, undeviatingly, undo,
   unerringly, universal, unlock, unmitigated, unqualified, unravel,
   unrelievedly, unreserved, unrestricted, unriddle, unscramble,
   untangle, untwist, unweave, up and down, up-and-down, upright,
   utter, utterly, valuate, value, verbally, verbatim,
   verbatim et litteratim, veritable, vertical, vertical circle,
   weigh, weight, wholesale, wholly, word by word, word for word,
   work, work out, zip up, zipper