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.
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   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.
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            He did not attempt to plumb his intellect. --Ld.
                                                  Lytton.
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   3. To seal with lead; as, to plumb a drainpipe.
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   4. To supply, as a building, with a system of plumbing.
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