The Collaborative International Dictionary of English v.0.48:

Supercargo \Su`per*car"go\, n. [Super- + cargo: cf. Sp.
   sobrecargo. Cf. Surcharge.]
   An officer or person in a merchant ship, whose duty is to
   manage the sales, and superintend the commercial concerns, of
   the voyage.
   [1913 Webster]

WordNet (r) 3.0 (2006):

supercargo
    n 1: an officer on a merchant ship in charge of the cargo and
         its sale and purchase

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

SUPERCARGO, mar. law. A person specially employed by the owner of a cargo to
take charge of the merchandise which has been shipped, to sell it to the
best advantage, and to purchase returning cargoes and to receive freight, as
he may be authorized.
     2. Supercargoes have complete control over the cargo, and everything
which immediately concerns it, unless their authority is either expressly or
impliedly restrained. 12 East, R. 381. Under certain circumstances, they are
responsible for the cargo; 4 Mass. 115; see 1 Gill & John. 1; but the
supercargo has no power to interfere with the government of the ship. 3
Pardes. n. 646; 1 Boulay-Paty, Dr. Com. 421.