The Collaborative International Dictionary of English v.0.48:

Argosy \Ar"go*sy\, n.; pl. Argosies. [Earlier ragusy, fr.
   ragusa meaning orig. a vessel of Ragusa.]
   A large ship, esp. a merchant vessel of the largest size.
   [1913 Webster]

         Where your argosies with portly sail . . .
         Do overpeer the petty traffickers.       --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

argosy
    n 1: one or more large merchant ships