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to cool (water) down; to make less hot (e.g. by adding cold water)
コーヒーを少しぬるめてから飲んだ。
I let the coffee cool down a bit before drinking.
熱すぎたスープを氷でぬるめた。
I cooled down the soup that was too hot with ice.
お茶が熱いので少しぬるめている。
The tea is hot, so I am cooling it down a little.