E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A Brazilian copper coin, worth about 2 1/2d.; also a round flat lump of lead used on board ship for playing quoits and chuck-penny. Hence dumpy or dumpty (squat or small). An egg is called a humpty-dumpty in the nursery verses beginning with Humpty Dumpty sat on a wall, etc.