A: Hurry up, or you'll be late again.
B: I'm coming.
A: You can't keep them cooling their heels each time.
B: Don't worry, I won't this time.
cool one's heels
Cool one's heels的意思是：让某人久等，让某人空等。
e.g. I was early for the appointment, so I had to cool my heels in the outer office.
e.g. I had to cool my heels for nearly an hour before the manager would see me.
e.g. Let him cool his heels in the outer office, that will teach him to be more polite.
e.g. The manager let the job applicants cool their heels in the outer office.
keep someone doing something：让某人做某事
e.g. I keep my son doing homework every evening.