dust and ashes used to convey a feeling of great disappointment or disillusion about something.

☞ Often found in the fuller form turn to dust and ashes in your mouth, the phrase is used in the Bible as a metaphor for worthlessness, for example in Genesis 18:27 and the Book of Job 30:19. It derives from the legend of the Sodom apple, or Dead Sea fruit, whose attractive appearance tempted people, but which tasted only of dust and ashes when eaten.