Heels Cooling Office I’m Loop Cool One’s Wait

1. The assistant had a 3 o’clock appointment with his boss, but the boss kept him cooling his heels in the outer office until well past 4:30.
2. I’m sorry I’m late getting home. The professor had me cooling my heels in his office while he was on the telephone.

The expression connotes some degree of annoyance and would usually be used in informal situations.