From Longman Dictionary of Contemporary Englishpour cold water over/on somethingpour cold water over/on somethingCRITICIZEto criticize someone’s plan, idea, or desire to do something so much that they no longer feel excited about it → pour
Examples from the Corpus
pour cold water over/on something• Clarisa picked him up and we poured cold water over his hand.• Arsenal were in the final, but Chapman poured cold water on hopes for the Double.• He started to pour cold water over me, inpart to staunch the blood, inpart to revive me.• Mieno is pouring cold water on the report before she's even seen it. 