I. plumb 1 /plʌm/ BrE AmE verb [transitive]
[ Date: 1500-1600 ; Origin: plumb 'metal weight on a plumb line' (13-21 centuries) , from Old French plomb 'lead' , from Latin plumbum ]
1 . plumb the depths (of despair/misery/bad taste etc) 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.
2 . to succeed in understanding something completely SYN fathom :
Psychologists try to plumb the deepest mysteries of the human psyche.
plumb something ↔ in phrasal verb
to connect a piece of equipment such as a washing machine to the water supply
II. plumb 2 BrE AmE adverb
1 . [always + adverb/preposition] informal exactly:
The bullet hit him plumb between the eyes.
2 . American English informal completely – often used humorously:
The whole idea sounds plumb crazy to me.
III. plumb 3 BrE AmE adjective technical
1 . exactly upright or level
2 . out of plumb not exactly upright or level