If someone says they're 'not about that life' it means that they don't or won't do something relating to that subject. It can be used in a variety of contexts and can be about a lot of things but it mostly used to reject someone inviting you to do something because you don't agree with it/don't like what they're doing/just don't want to.
It can also be edited to fit the purpose more by adding in a word before the word life to describe what kind of life said person is not about.
Example 1-
Person 1: Hey, I'm having a Sexy Party this weekend, wanna come?
Person 2: Nah, I'm not about that life. I'm sure you'll be coming enough for the both of us.
Example 2-
Person: Want to take a picture with me?
Person 2: Nope, I'm not about that selfie life.
Another way to say that something doesn't fit a person, doesn't suit them, or isn't met for them. Commonly pronounced as "ain't 'bout dat life."
Sarah is so hot for Dave that she is acting like she's easy to get his attention, but she ain't about that life.
That geek Bill Nye watched way too much Breaking Bad and now he's trying to find a partner to cook meth with. We all know he ain't 'bout dat life.
She was talking shit about my best friend, so i punched her in the face instead of talking shit. I ain't fake, I ain't 'bout dat life.
bout that life originally referred to the florida law that gives 10 years for possession, 20 for firing, and life inprisonment for killing with a concealed weapon. if you are bout that life, you are willing to serve life in prison for killing a person. Later, bout that life was adapted to mean things such as being brave, popular, or 'down' for the respective crew.
The definition of you ain't about that life means that your not about to do something. You claim that you are but you don't, your to afraid and chicken out.
So your really gonna rob the bank? Nahh you ain't about that life, dog.