Meaning of "AB"

I'm currently watching Truth be told

in the second episode there are two sentences that mention "AB".

I think he's innocent.

So what? He's AB, Poppy.

then a second time

Cave is housed in San Quentin, right?

Yeah

So he's AB.

What does AB exactly means?

Solution 1:

Aryian brotherhood. Aka white boys

It’s Aryan Brotherhood, Poppy’s husband says the term in episode 3.