The definitions listed are cargo, or part of ship, or person which is useless, unnecessary, superfluous, non-functional. Also, in reference to the captain, lacking authority, or incompetent.
I ran across a reference, I don't recall where, it started out its life as "studding sail". Over time the words, like other ancient nautical words like focsel or focsle (forecastle [fo'c'sle, fo'c's'le), suffered from what I call the Telephone Game Affect. After a number of iterations it went through a variety of mutations such as, studsail, stunsail, dunsail, stunsel, and dunsel.
Studding sails were used on old square rigged sailing ships. They were an attempt to capture more wind when there was virtually none. Crew weren't enamoured with them as setting them was a pain.
Studsails were attached to the ends of the yards to add canvas to the sides of existing sails. Having to go out to the end if a yardarm to set them was difficult. Worse was having to bring them in when the wind picked up, the difficulty making it dangerous.
There was a very narrow nich in wind speed where they were useful. They had to be brought in when decent wind was found otherwise it could stress the rigging to the breaking point.
Most crew thought they were useless, worthless, unnecessary, non-functional bit of fluff. In light winds they just hung there with the rest of the canvas. There were only certain low winds speeds where they would fill and actually contribute. Most thought they weren't worth the effort.
I can't find the reference I discovered long ago and haven't found anything concrete to substantiate this. Does anyone have anything substantive on the etymology of "dunsel"?