at the drop of a hat

[at the drop of a hat] {adv. phr.}, {informal} 1. Without waiting; immediately; promptly.

If you need a babysitter quickly, call Mary, because she can come at the drop of a hat.

Compare: ON THE SPUR OF THE MOMENT.

2. Whenever you have a chance; with very little cause or urging.

At the drop of a hat, he would tell the story of the canal he wanted to build.

He was quarrelsome and ready to fight at the drop of a hat.