A dumby is a hand that is dealt for a card game, but has no one to play it. It is commonly modernized to 'dummy', but the traditional form is used particularly in the games of Whist and Loo. In fact, was once common to refer to game of Three-Handed Whist simply as Dumby and Two-Handed Whist as Double Dumby.
The dumby is dealt face up on the table, and is either played by the person who would be the partner of the 'player' in the dumby position (as in Whist), or is played by a person who has passed that round (as in Loo).