In One's Dreams Meaning
(idiomatic, colloquial) Used to express the speaker's belief that a preceding statement expressed a desired rather than an actual state of affairs.
Example: "Can't go? Why not? Got a hot date tonight?" / "In my dreams! No, I've got to visit a friend in the hospital."
"They said the economy will boost sales next quarter." / "In their dreams! Maybe next year the economy will recover."