cast stones against the wind
To do something that is futile or will not work out. Trying to get an A in Chem class now, after blowing off your assignments all semester, is like casting stones against the wind. You're doomed! It's too late to repair my relationship with my mother. I might as well cast stones against the wind. There's no way Bill is gonna lend you money again. You can ask him, but you're casting stones against the wind.
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