plumb¹ / pləm/ • v. [tr.] 1. measure (the depth of a body of water). ∎  [intr.] (of water) be of a specified depth: at its deepest, the lake scarcely plumbed seven feet. ∎  explore or experience fully or to extremes: she had plumbed the depths of depravity.2. test (an upright surface) to determine the vertical.• n. a plumb bob.• adv. 1. inf. exactly: a contrabassoonist who sits plumb in the middle of the wind section. ∎  to a very high degree; extremely: they must both be plumb crazy. ∎  completely: the transmission was plumb worn out.2. archaic vertically: drapery fell from their human forms plumb down.• adj. vertical: ensure that the baseboard is straight and plumb.PHRASES: out of plumb not exactly vertical: the towers are inclined, from four to ten feet out of plumb.plumb² • v. [tr.] install and connect water and drainage pipes in (a building or room): the house could not be plumbed at all.

plumb ball of lead attached to a line. XIII (now familiar chiefly in phr. out of p. out of the vertical, and p.-line XVI). ME. plumbe, prob. — OF. *plombe, repr. by plomme sounding-lead:- Rom. *plumba; later assim. to (O)F. plomb lead:— L. plumbum, of obscure orig.
Hence plumb adj. vertical, adv. vertically XIV; plumb vb. sound with a plummet XVI.