Cain, mark of: The mark that God set upon Cain, the eldest son of Adam. According to the Book of Genesis in the Bible, God rejected Cain's grain offering while accepting the animal offering of his brother Abel. Out of jealousy, Cain murdered his brother and was compelled by God to wander the earth as "a fugitive and a vagabond." Lest someone slay Cain and end his misery, "the Lord set a mark on Cain."
The mark of Cain now refers to a person's sinful nature.