Plumb  —n. Lead ball, esp. Attached to the end of a line for finding the depth of water or testing whether a wall etc. Is vertical. —adv. 1 exactly (plumb in the centre). 2 vertically. 3 us slang quite, utterly (plumb crazy). —adj. Vertical. —v. 1 a provide with plumbing. B (often foll. By in) fit as part of a plumbing system. C work as a plumber. 2 sound or test with a plumb. 3 reach or experience (an extreme feeling) (plumb the depths of fear).