Source : Webster's Revised Unabridged Dictionary (1913)

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

   {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.