The Collaborative International Dictionary of English v.0.48:

Plunderage \Plun"der*age\, n. (Mar. Law)
   The embezzlement of goods on shipboard. --Wharton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

plunderage
    n 1: the act of plundering (especially the embezzlement of goods
         on shipboard)

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

PLUNDERAGE, mar. law. The embezzlement of goods on board of a ship, is known
by the name of plunderage.
     2. The rule of the maritime law in such cases is, that the whole crew
shall be responsible for the property thus embezzled, because there must be
some negligence in finding out the depredator. Abbott on Ship. 457; 3 John.
Rep. 17; 1 Pet. Adm. Dee. 243; 1 New Rep. 347; 1 Pet. Adm. Dee. 200, 239.