The Collaborative International Dictionary of English v.0.48:

Portoise \Por"toise\, n. [Perhaps fr. OF. porteis portative,
   portable.] (Naut.)
   The gunwale of a ship.
   [1913 Webster]

   To lower the yards a-portoise, to lower them to the
      gunwale.

   To ride a portoise, to ride an anchor with the lower yards
      and topmasts struck or lowered, as in a gale of wind.
      [1913 Webster]