An argosy is a merchant vessel, especially one richly laden.
The term dates to the sixteenth century, to a period when such vessels plied a brisk trade with England from a certain port in Italy.
That port was Ragusa (now in Yugoslavia and called Dubrovnik).
A ship hailing therefrom was, of course, “a Ragusa,” a term rapidly degenerating in dockside language into “a Ragusy,” and hence to argosy.
In The Merchant of Venice Shakespeare has:
There where your Argosies with portly saile
Like Signiors and rich Burgers on the flood
Or as it were the Pageants of the sea,
Do ouer-peere the pettie Traffiquers
That curtsie to them, do them reuerence
As they flye by them with their wouen wings.