promise someone the moon
promise (one) the moon
To promise one something that cannot be done. Your clients will come to distrust you if you keep promising them the moon.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
promise someone the moon (or earth)
promise something that is unattainable. British 1998 New Scientist Scientists tend to promise taxpayers the moon, and then not deliver.
Farlex Partner Idioms Dictionary © Farlex 2017