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.
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.
3. To seal with lead; as, to plumb a drainpipe.
4. To supply, as a building, with a system of plumbing.