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.
Death saw two players playing cards, But the game was not worth a dump.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894