Real Dictionary

plumb
If something is plumb in a particular place, it is exactly in that place.
verb: If you plumb something mysterious or difficult to understand, you succeed in understanding it.
verb: When someone plumbs a building, they put in all the pipes for carrying water.

plumb in American English
a lead weight (plumb bob) hung at the end of a line (plumb line), used to determine how deep water is or whether a wall, etc. is vertical
in a vertical direction; straight down
perfectly vertical; straight down
noun: a small mass of lead or other heavy material, as that suspended by a line and used to measure the depth of water or to ascertain a vertical line

plumb in British English
adjective: (intensifier)
adverb: in a vertical or perpendicular line
noun: a weight, usually of lead, suspended at the end of a line and used to determine water depth or verticality
verb: to test the alignment of or adjust to the vertical with a plumb line