at the drop of a hat

From the customary starting of competitions by making a sweeping downward motion with a cap.

• "We're expected to just do it at the drop of a hat."
• "If you need help, just call on Mike. He can come at the drop of a hat."
• "A tremendous thing happened when Dean met Carlo Marx. Two keen minds that they are, they took to each other at the drop of a hat."
• "Let's face it. We are swollen up like an overfed slug, our breath stinks, the tiniest irritant is magnified ten thousand times, and we cry at the drop of a hat."

• at a moment's notice
• at once
• in a heartbeat
• off the cuff