The Collaborative International Dictionary of English v.0.48:

Deadwood \Dead"wood`\, n.
   1. (Naut.) A mass of timbers built into the bow and stern of
      a vessel to give solidity.
      [1913 Webster]

   2. Dead trees or branches; useless material.
      [1913 Webster]

   3. [fig.] People who are unproductive; -- used especially in
      reference to employees.
      [PJC]

WordNet (r) 3.0 (2006):

deadwood
    n 1: a branch or a part of a tree that is dead
    2: someone or something that is unwanted and unneeded [syn:
       fifth wheel, deadwood]