If someone leaves you to cool your heels, they keep you waiting for a long time. “让某人的脚后跟都受凉了”，cool one’s heels表示让某人等候多时。

Examples:

Do not make your guests cool their heels when you have appointments with them. 同客人订了约会就不该让人家久等。

Fans of the band were forced to cool their heels outside the theater for 90 minutes. 乐队的歌迷被迫在场外久等了90分钟。