The Collaborative International Dictionary of English v.0.48:

Plumb \Plumb\ (pl[u^]m), n. [F. plomb, L. plumbum lead, a leaden
   ball or bullet; cf. Gr. mo`lybos, mo`libos, mo`lybdos. Cf.
   Plummet, Plunge.]
   A little mass or weight of lead, or the like, attached to a
   line, and used by builders, etc., to indicate a vertical
   direction; a plummet; a plumb bob. See Plumb line, below.
   [1913 Webster]

   Plumb bob. See Bob, 4.

   Plumb joint, in sheet-metal work, a lap joint, fastened by
      solder.

   Plumb level. See under Level.

   Plumb line.
   (a) The cord by which a plumb bob is suspended; a plummet.
   (b) A line directed to the center of gravity of the earth.

   Plumb rule, a narrow board with a plumb line, used by
      builders and carpenters.
      [1913 Webster]

WordNet (r) 3.0 (2006):

plumb rule
    n 1: a plumb line attached to a narrow board