心神耗弱
shin.shin.kou.jaku

Literally: heart – soul – decrease – weak

Alternately: Decreased mental capacity. Having lost the ability to judge right and wrong and act accordingly. This is actually a legal and medical term indicating diminished responsibility: that someone cannot be held fully responsible for their actions because of the effects of drugs, mental disorders, or similar factors. (Note, however, that the proper response to this state is not “give the person even more power”!)

Notes: This is a compound of compounds: 心神 is “mind,” “spirit,” or even “intent”; 耗弱 is a weakened state caused by something being worn down or degraded over time.

In some situations 耗 may be read as mou, but for the purposes of this yojijukugo, only kou is considered valid.

A list of notable cases in which this resulted in a reduced sentence. Source = Mainichi Shinbun.