The term 'knee jerk, perhaps' can be summarized as a SPASM, which is a five-letter word. A SPASM is an involuntary contraction of a muscle or group of muscles, often caused by a sudden stimulus. Knee jerk reactions are a type of SPASM, where a person reacts to a stimulus without thinking. This type of reaction is often seen as a negative behavior, as it does not allow for thoughtful consideration of the situation.