Though the 20th century was a time of great enlightenment, it was also an era of many of man's most grievous crimes. The adjective grievous means extremely sad or hurtful and is closely related to the verb "grieve."
Grievous is derived from the noun grief, which means "sadness." When something is grievous, therefore, it's not just bad, but so bad that it causes unusually intense sadness. It's often used to describe crimes and other bad deeds that go beyond mere criminality and into the world of cruelty. For example, it's harmful to steal from someone; it's grievous to steal from them and then, having stolen their stuff, beat them up or kill them.
