I’m not responsible for someone else
—When I asked Roger if he knew why his roommate was late to class, he answered, “Don’t ask me. Am I my brother’s keeper?” 

After leaving the Garden of Eden, Eve had two sons, named Cain and Abel. One day the brothers presented offerings to God. While God accepted Abel’s offering, he did not accept Cain’s. Cain then became angry and violent:

Cain said to his Abel, “Let’s go out to the field.” While they were in the field, Cain attacked his brother and killed him.

Then the Lord said to Cain, “Where is your brother Abel?” And he replied, “I don’t know! Am I my brother’s guardian? [Am I my brother’s keeper?—KJV]” But the Lord said, “What have you done? The voice of your brother’s blood is crying out to me from the ground! So now, you are banished from the ground, which has opened its mouth to receive your brother’s blood from your hand. When you try to cultivate the ground it will no longer yield its best for you. You will be a homeless wanderer on the earth.”

. . . .

So Cain went out from the presence of the Lord and lived in the land of Nod, east of Eden. (Genesis 4:8-12,16)

When Cain asked, “Am I my brother’s keeper?” he already believed he was not responsible for Abel. This is how the phrase is most often used now, along with simply saying, “I am not my brother’s keeper.”

To raise Cain means “to cause a disturbance” or “to lose one’s temper,” as if the person has called up the spirit of Cain.

Jonathan Swift was the first to use “Land of Nod” in print as a pun for “sleep”—for nodding one’s head while falling asleep or “nodding off”—in A Complete Collection of Genteel and Ingenious Conversation  (1738). In the Genesis story, Nod is a Hebrew word meaning “wandering.”