ask for the moon
Make an unreasonable demand, request the unattainable, as in $1,000 for her birthday? Mary might as well be asking for the moon. This hyperbolic idiom appeared in the mid-1800s in slightly different form. Charles Dickens had it as cry for the moon (in Bleak House, 1852) and William Makepeace Thackeray as wish for the moon (in Lovell the Widower, 1860). Today ask is the most common version.