flight of fancy
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.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.