The Collaborative International Dictionary of English v.0.48:

Spun \Spun\ (sp[u^]n),
   imp. & p. p. of Spin.
   [1913 Webster]

   Spun hay, hay twisted into ropes for convenient carriage,
      as on a military expedition.

   Spun silk, a cheap article produced from floss, or
      short-fibered, broken, and waste silk, carded and spun, in
      distinction from the long filaments wound from the cocoon.
      It is often mixed with cotton.

   Spun yarn (Naut.), a line formed of two or more rope-yarns
      loosely twisted.
      [1913 Webster]

WordNet (r) 3.0 (2006):

spun yarn
    n 1: (nautical) small stuff consisting of a lightweight rope
         made of several rope yarns loosely wound together