promise the moon 做无法兑现的许诺

promise (somebody) the moon 做无法兑现的许诺（moon也可以换成earth）。

to make promises that will be impossible to keep

例句：

I can't promise you the moon, but I'll do the best job I can.

我不对你胡乱许诺，不过我会尽力而为的。