When the weather is cool, it's not quite cold but not warm either. When people are cool, they're mellow and in control.
Cool has many meanings. When the weather goes from 70 degrees to 50, it cooled off. A winning pool player who starts losing also cools off. Drinking lemonade on a hot day is yet another way to cool off. When you lose your temper, you lose your cool. Hot cookies should cool before you eat them. Something fashionable is cool, but if you're cool to an idea, you're not so sure about it. Any situation that becomes less intense has cooled.