Akugyaku was one of hachi-gyaku (eight unpardonable crimes) defined by the ritsuryo (legal codes of the Nara and Heian periods). It refers to the wounding or killing of the parent(s)/ grandparent(s) or the killing of uncle(s)/ aunt(s), maternal grandparent(s)/elder brother(s)/elder sister(s). In female cases it also included killing their husband and his parent(s).
Among hachi-gyaku it ranked fourth and was regarded as the most serious crime among the problems involving a family. Apart from the gravity of a crime, all social privileges were deprived and also no amnesty was granted.
The term "akugyaku-mudo," which is used today to describe cruel act against humanity, is derived from it.