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."