knee-jerk reaction
nounUK /ˌniː ˈdʒɜːk riˈækʃn/
US /ˌniː ˈdʒɜːrk riˈækʃn/
A quick answer you give without thinking first.
When the teacher asked if I did my homework, my knee-jerk reaction was to say yes before I checked.
OTHER MEANINGS
2. A sudden physical movement of the leg caused by a tap on the knee.
The doctor tapped my knee and got a knee-jerk reaction.
3. An immediate opinion or decision made without careful thought.
His knee-jerk reaction to the new rule was to complain before reading it.
4. A fast, unplanned response to something surprising or upsetting.
Her knee-jerk reaction to the loud noise was to jump back.