What's the meaning of the phrase 'Keel over'?
To keel over is to fall, suddenly and in a heap.
What's the origin of the phrase 'Keel over'?
'Keel over' was originally a nautical expression. The keel is the longitudinal timber to which the rest of the boat's timbers are fixed, usually seen as a ridge along the middle of the hull. 'Keeled over' referred to ships which had capsized or were laid on their side on land, either way with their keels showing.
The phrase originated in the mid-19th century. The earliest reference I can find of it in print is in the Philadelphia Public Ledger, August 1837:
We are informed that large quantities of fish are daily pouring from the Lake into the Basins, where they "keel over" and die off in no time.
It's clear from the figurative use above that the phrase must have already been known as a direct reference to actual keels pointing into the air. However, so far, I can't find that in print.
The earliest example in print that explicitly refers to ships is from around the same time. This time from England, in the Morning Post newspaper, January 1838:
The Dreadnought's moorings gave way ahead, and she was slewed halt round by the tide, and the heavy masses of ice which came down upon her caused great alarm for the safety of those on board, she being the hospital ship. It was expected that she would get aground at low water and be keeled over.