ADJ
未经思考作出的；依靠本能作出的
If you call someone's response to a question or situation a knee-jerk reaction, you mean that they react in a very predictable way, without thinking.
例句
The knee-jerk reaction to this is to call for proper security in all hospitals.
对此类状况的第一反应会是要求所有医院都采取适当的安全措施。