At the Drop of a Hat!

The origin probably comes from the C.18th when the dueling rules required a "signal" to be made to start firing. People used to drop a handkerchief.
The duels in C.19th were started at the signal of dropping a hat. So, dropping a hat meant GO!

The meaning of "at the drop of a hat" is to do something suddenly, without hesitation.

For example: I can't go climbing at the drop of a hat. I need time to gather what I might need, such as water, flashlight and like so on.

Dan always liked go fishing. He's always been ready to do it at the drop of a hat.

Got it?!