Adjective: To end in failure. (Related to the expression down the tubes
1. To pull the chain on an old fashioned elevated toilet
tank in order to flush fecal matter from the commode.
2. Flushing something down the toilet so that it ends up in the septic
3. When a prototype ship model design fails hydrodynamic testing in a water-filled tank.
4. Naval aviators who fail to pass the dunker
(Helicopter Underwater Escape Training) which is conducted in a large water tank.