The term originated in the 1907 Hague Convention preamble, this codified the customary law of armed conflict. This codification was based on existing State practices that derived from those values and principles deemed to constitute the “laws of humanity,” as reflected throughout history in different cultures.
Crimes Against Humanity - the 1907 Hague Convention
The term crimes against humanity have come to mean anything atrocious committed on a large scale. This is not, however, the original meaning nor the technical one.