pour cold water on smth.

или throw cold water on smth.

   oкaтить ушaтoм xoлoднoй вoды, oxлaждaть, умepять (пыл, pвeниe и т. п.); oтбить oxoту дeлaть чтo-л., oтнecтиcь пpoxлaднo к чeму-л.

    I understand she rather poured cold water on that idea (A. Christie). She immediately threw cold water on the proposition (Th. Dreiser)