1
general::
An unrealistic idea or fantastic notion, a pipe dream. For example, She engaged in flights of fancy, such as owning a million-dollar house. This idiom uses flight in the sense of "a soaring of the imagination," a usage dating from the mid-1600s.
American Heritage Idioms
2
general::
Phrase(s): flight of fancy
an idea or suggestion that is out of touch with reality or possibility. • What is the point in indulging in flights of fancy about exotic vacations when you cannot even afford the rent?
McGrawhill's American Idioms And Phrasal Verbs