(3 syl.) The word is used to express the uncertainty of things on which we set our heart. As the beggar looked forward to a feast, but found only empty dishes; so many a joy is found to be mere illusion when we come to partake of it.
To-morrow! the mysterious unknown guest Who cries aloud, “Remember Barmecide!” And tremble to be happy with the rest.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Barmecide from Infoplease: