The Collaborative International Dictionary of English v.0.48:

Dunnage \Dun"nage\, n. [Cf. Dun a mound.] (Naut.)
   Fagots, boughs, or loose materials of any kind, laid on the
   bottom of the hold for the cargo to rest upon to prevent
   injury by water, or stowed among casks and other cargo to
   prevent their motion.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

DUNNAGE, mer. law. Pieces of wood placed against the sides and bottom of the
hold of a vessel, to preserve the cargo from the effect of leakage,
according to its nature and quality. 2 Magens, 101, art. 125, 126 Abbott on
Shipp. 227.