|Origin:||plumb 'metal weight on a plumb line' (13-21 centuries), from Old French plomb 'lead', from Latin plumbum|
to feel an unpleasant emotion in a very extreme way, or to behave in a way that is extremely unpleasant or morally bad:
When his wife left him, Matt plumbed the very depths of despair.
That night they plumbed the depths of treachery and horror, and murdered the king as he slept.
to succeed in understanding something completely [= fathom]:
Psychologists try to plumb the deepest mysteries of the human psyche.
plumb something ↔ inphrasal verb
to connect a piece of equipment such as a washing machine to the water supply