ˈknee-jerk BrE AmE adjective [only before noun]
a knee-jerk reaction, answer etc is what you feel or say about a situation from habit, without thinking about it SYN automatic :
A victim’s knee-jerk reaction to the crime is often revenge.
KNEE-JERK
Meaning of KNEE-JERK in English
Longman Dictionary of Contemporary English. Longman - Словарь современного английского языка. 2012