The phrase "to keel over" means to fall or collapse in a helpless or lifeless manner, often used figuratively to describe someone who is very weak or exhausted. It can also be used literally to refer to something that has fallen or collapsed, such as an object or structure. The origin of the phrase comes from the nautical term "to keel," which means the bottom of a ship's hull. When a sailor would "keel over" it meant they had died and their body was lying flat on its back with its face up, giving the appearance that they were resting on their keel or bottom side.