Source : Webster's Revised Unabridged Dictionary (1913)

Oakum \Oak"um\, n. [AS. [=a]cumba; pref. ? (cf.G. er-, Goth.
   us-, orig. meaning, out) + cemban to comb, camb comb. See
   {Comb}.]
   1. The material obtained by untwisting and picking into loose
      fiber old hemp ropes; -- used for calking the seams of
      ships, stopping leaks, etc.

   2. The coarse portion separated from flax or hemp in
      nackling. --Knight.

   {White oakum}, that made from untarred rope.

Source : WordNet®

oakum
     n : loose hemp or jute fiber obtained by unravelling old ropes;
         when impregnated with tar it was used to caulk seams and
         pack joints in wooden ships