Pour/throw cold water on something
Definition 1 of 1
Pro Tip 1/3Metaphorical Use
The phrase is not about literal water, but symbolizes discouragement or negativity. 
The critic threw cold water on the new play, calling it uninspired.
Pro Tip 2/3Negative Emotion
It often expresses a strong negative reaction or opinion. 
When I suggested working late, my team poured cold water on the idea.
Pro Tip 3/3Idiomatic Caution
Use this phrase carefully; it can sound blunt or harsh if misapplied. 
Be mindful not to pour cold water on someone's dreams without a gentle approach.