Meaning of "at the drop of a hat"

[Anchela]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #41 "Talking through his hat", question 7

You simply can't trust that politician because he changes his mind every five minutes at the ......... of a hat.

(a) fall
(b) tumble
(c) jump
(d) drop

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #41 "Talking through his hat", answer 7

You simply can't trust that politician because he changes his mind every five minutes at the drop of a hat.

Correct answer: (d) drop

Your answer was: correct
_________________________

I don't understand the phrase "at the drop of a hat".
Could you explain it to me?

Thank you
Anchela

[Conchita]

at the drop of a hat
If you do something at the drop of a hat, you do it immediately without stopping to think about it:

People will file lawsuits at the drop of a hat these days.

(Cambridge Advanced Learner's Dictionary)