Scupper

A drain or opening that carries water from a deck, cockpit or other exposed area overboard or into a drainage system.

A scupper may pass directly through the vessel’s side, connect by hose to an outlet or feed a freeing port or drain manifold. Its capacity and height relative to the waterline determine whether it can clear rain, spray or shipped water without allowing unacceptable backflow.

Leaves, lines, ice and debris can block scuppers, while failed hoses or submerged outlets can create flooding paths. Gratings, non-return devices, hose condition and seacocks where fitted require inspection, but a check valve should not be assumed to make a low drain fail-safe.

Slang or Saying

Naval origin theory: In naval slang, “to scupper” someone meant to kill or dispose of them, possibly by throwing them overboard through a scupper (not pleasant!). That violent origin may have extended metaphorically to mean to destroy or sink something — like an idea, plan, or project.
Broader metaphorical use: Since scuppers were associated with damage control and dangerous overflow (i.e., water coming onto the deck), the term might’ve picked up a more destructive or ruinous connotation over time — especially in British naval slang.