The bilge is the lowest compartment on a ship, below the waterline, where the two sides meet at the keel. The bilge is the lowest level of the ship. It’s loaded with ballast and slimy, reeking water.
The word is sometimes also used to describe the water that collects in this area. Water that does not drain off the side of the deck drains down through the ship into the bilge. This water may be from rough seas, rain, leaks in the hull or stuffing box, or other interior spillage. The collected water must be pumped out to prevent the bilge from becoming too full and threatening to sink the ship. The water that collects is often noxious, and "bilge water" or just "bilge" has thus become a derogatory colloquial term used to refer to something bad, fouled, or otherwise offensive.
The term "bilged" refers to the deliberate or accidental flooding of the bilge which incapacitates the ship, slows its speed, and/or to makes it difficult or impossible to handle. This process may be carried out intentionally by enemy action or by the crew of the ship itself in order to save it from falling into enemy hands. Taking this action may allow the ship to be recovered later by being pumped out and re-floated during a high tide. When done on purpose it is normally referred to as scuttling. When done accidentally by grounding, it usually resulted in the ship being entirely lost.