the.com/stern post

means The sturdy upright timber or structural beam at the very back of a ship's hull, forming the frame to which the rudder is attached.

from A straightforward compound: 'stern,' meaning the rear of a vessel (likely from Old Norse 'stjórn,' meaning steering — since the stern is where steering gear lived long before rudders as we know them existed), plus 'post,' a thick upright support, from Latin 'postis.' Shipwrights needed something solid to hinge a rudder on, so they named it for exactly what it is and where it stands.