A lead weight attached to a line and used to indicate a vertical direction. out ofplumb or off plumb. : out of vertical or true. plumb. adverb.

Plumb as a noun refers to a lead weight that’s attached to a line and used especially to indicate a vertical direction. … Unlike plum, plumb hasn’t been satisfied with simple noun use. No, the word functions also as an adverb, a verb, and an adjective.