'To ride a portoise' definitions:

Definition of 'To ride a portoise'

From: GCIDE

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]