[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./