To pour cold water on something is to dampen enthusiasm for an idea. He poured cold water on the whole plan.
It is done to an idea rather than to a person, and it can be sensible as well as unkind. The picture is exactly what it says.
1phrasepour cold water onTo dampen somebody's enthusiasm for an idea.
“The finance director will pour cold water on any of it.”
pour cold water on is a phrase.
Practised in Idioms