It's a memorable quote in the bible found in Genesis 4:9, meaning:
1. Are you responsible for how you respond to your brother?
2. Are you held accountable for how you treat your fellow man?
It's really a sarcastic question/remark from Cain to God after Cain killed his brother, Abel, to keep from admitting to the fact that he did kill Abel.
Nino Brown: To CMB. On and on. Am I my brother's keeper?
a term used by women to say they want to be friends with you, but nothing more. If your girl says this it meens you don't and will never have a shot with her unless you man up and take a stand.