knee–jerk 
/ˈni:ˌʤɚk/ adjective
always used before a noun /ˈni:ˌʤɚk/ adjective [more knee–jerk; most knee–jerk] 1 : occurring quickly and without thought
▪ It was a knee-jerk [=automatic] reaction. 2 disapproving : often reacting quickly and without thought
▪ He said that they were just a bunch of knee-jerk liberals.
▪ It was a knee-jerk [=automatic] reaction.
▪ He said that they were just a bunch of knee-jerk liberals.
