To lose or cause to lose the heat of any deep feeling (as love, enthusiasm, or anger); make or become calm, cooled or indifferent; lose interest.
A heated argument can be settled better if both sides cool down first.
John was deeply in love with Sally before he left for college, but he cooled off before he got back.
Their friendship cooled off when Jack gave up football.
The neighbor's complaint about the noise cooled the argument down.