a flight of fancy

a flight of fancy/fantasy/imagination

an idea which shows a lot of imagination but which is not practical or useful in real situations You were talking about cycling across the US, or was that just another flight of fancy?
See also: fancy, flight, of
References in classic literature ?
I answered, smiling; and then, with rather a flight of fancy, "It's the idea of a garden in the middle of the sea.
It was, in truth, as impossible for him to take a flight of fancy as it would be for a watchmaker to put together a chronometer with nothing except a two-pound hammer and a whip-saw in the way of tools.
BIRMINGHAM art lovers are invited to go on a flight of fancy.