At the drop of a hat
Definition 1 of 1
Pro Tip 1/3Quick Decision
Used when someone makes a choice quickly, without much thought or hesitation. 
She agreed to join the game at the drop of a hat.
Pro Tip 2/3Immediate Action
Indicates someone is willing to do something right away, showing urgency or eagerness. 
If you need help, I'll be there at the drop of a hat.
Pro Tip 3/3Informal Tone
It's a casual phrase, used more often in informal conversations among friends or family. 
He's ready to debate at the drop of a hat, especially on sports topics.