1. 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.

    e.g. 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。