A little mass of lead, or the like, attached to a line, and used by builders, etc., to indicate a vertical direction; a plummet; a plumb bob (UK); a plumb line (US)
adjust with a plumb line so as to make vertical examine thoroughly and in great depth measure the depth of something weight with lead exactly vertical; "the tower of Pisa is far out of plumb"
If you plumb something mysterious or difficult to understand, you succeed in understanding it. She never abandoned her attempts to plumb my innermost emotions = fathom
If someone plumbs the depths of an unpleasant emotion or quality, they experience it or show it to an extreme degree. They frequently plumb the depths of loneliness, humiliation and despair
Plumb is the opposite of level Holding a level perpendicular to the ground is plumb Walls are said to be plumb and floor systems are referred to as level
If you say that something plumbs new depths, you mean that it is worse than all the things of its kind that have existed before, even though some of them have been very bad. Relations between the two countries have plumbed new depths