Green’s Dictionary of Slang

a dismissive excl.: don’t you wish!

1987		(con. 1967) E. Spencer Welcome to Vietnam (1989) 19: I thought at the time that he was different than I. Dream on, Ernest.
1991		P. Cornwell Body of Evidence (1992) 175: ‘I don’t suppose he’s a journalist for some tabloid,’ I said in jest. ‘Yo. Dream on.’.
1999		Observer Screen 25 July 6: ‘No, Mr Bond,’ replied the rotund villain. ‘I expect you to die!’ Dream on. Mr Bond didn’t die.
2000		Guardian Travel 29 Jan. 19: Well, dream on.