The Collaborative International Dictionary
To ride a portoise
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.