At the drop of a hat

Thành ngữ này đã xuất hiện từ những thập niên 80 vào cái thời mà người Mỹ rất thích hay đội nón, và trước khi khởi đầu một cuộc chạy đua, họ phải vứt ngay cái nón xuống đất để chạy cho nhanh. Chính vì thế nghĩa của cụm thành ngữ này có nghĩa là "làm điều gì đó ngay tức khắc mà không bị ai xúi giục hay bắt buộc".

Meaning

• do something immediately without stopping to think about it.
• without hesitation, quickly
• instantly, on the slightest signal or urging
• suddenly without any planning and for no obviously reason.
• do it suddenly and easily, often without any preparation 

Examples

• If you need anything, just call me. I'll be there at the drop of a hat.
• My boss want me to come to work at the drop of a hat.
• Jack was small for his age but he had a quick temper and was always getting into fights with the other boys at the drop of a hat. Mother and dad used to worry about this, but he learned to control his temper by the time he got to high school.
• He'd buy her expensive clothing at the drop of a hat and worry about how he would pay for it later. 
• We now have a situation where laws may be changed at the drop of a hat.