Definition of the word keel over

The phrase "keel over" originated from the nautical world, specifically, the anatomy of ships. The keel is the lowest and longest plank that forms the base or centerline of a ship. If a ship were to "keel over," it would literally mean that it has tilted or capsized to such an extent that its keel has submerged, causing the ship to become unseaworthy. The usage of "keel over" in figurative contexts began in the second half of the 1700s. At that time, the idiomatic phrase was "to loll over," which meant "to droop," "to fail," or "to become powerless." "To keel over" is believed to have been a contraction of "to fall over as if the keel has sunk," implying a sudden or dramatic collapse or demise, both for the ship and, by extension, for people. In common parlance today, "to keel over" often signifies a sudden but temporary loss of consciousness, probably caused by dizziness, exhaustion, or immoderate emotions. Technically, it should mean a tilt so severe that a person collapses, but the phrase is often used more loosely to suggest a fall or collapse.