The phrase my brother's keeper is a reference to the Biblical story of Cain and Abel in the book of Genesis. It is generally understood to mean "responsible for the welfare of my brother," or, by extension, of other human beings in general. Cain, the first murderer according to the Bible, eschewed this responsibility, but the phrase is often used with the suggestion that those who place their faith in the Bible do have such a responsibility to their fellow man.

The story of Cain and Abel appears in the first 16 verses of the fourth chapter in the Book of Genesis. The phrase my brother's keeper appears in William Tyndale's 1530 English translation, which was incorporated in the King James Version of the Bible.

Cain and Abel were the sons of Adam and Eve. Cain, a farmer, and Abel, a shepherd, both made sacrifices of the fruit of their labor to God. God looked favorably upon Abel's sacrifice, but not Cain's. In his anger over the incident, Cain murdered his brother.

God asked Cain where his brother was, and Cain replied, in the Tyndale translation, "I don't know. Am I my brother's keeper?" God knew that Cain had killed Abel and punished the murderer by making him a "restless wanderer." When Cain complained that anyone who came across him would kill him because of his actions, God declared that "whoever kills Cain will suffer vengeance seven times over" and marked Cain as a sign that he was not to be harmed.

With his question, "Am I my brother's keeper?" Cain attempted to throw the scent off his misdeed by claiming no responsibility regarding his brother. Followers of biblical teachings often interpret this story as a reminder that they are responsible for the welfare of others, so my brother's keeper has come to refer to a person who looks out for and cares for others, whether literally brothers or sisters or not. A "brother's keeper" may donate his or her time or resources to help others and place the needs of others before his or her own, for example.