Green’s Dictionary of Slang

A.B. n.

[abbr.]

1. (N.Z.) the menstrual period [abbr. Annie Brown, on the model of menstruation = a ‘visiting’ woman].

[NZ]McGill Dict. of Kiwi Sl. 9/1: AB a woman’s period, short for ‘Annie Brown’; eg ‘I’ll have to cry off tennis today, Maureen. Old AB’s visiting.’.
[NZ]McGill Reed Dict. of N.Z. Sl. [as cit. 1988].

2. (US) fools as a group, lit. Asshole Brigade [asshole n. (3)].

[US]S. King Christine 37: ‘Oh yeah, I got some neighbours straight out of the old AB.’ ‘What’s that?’ I asked him. ‘The Asshole Brigade, son.’.

3. (US prison) Aryan Brotherhood, the white supremacist organization that unites many white convicts during (and after) their time in jail.

Hearings [...] of the House Committee on Internal Security 1237: The Mexican Mafia (EME), Nuestra Familia, and the Aryan Brotherhood (AB).
R.M. Carter et al. Correctional Institutions 151: Prison gangs are frequently characterized as violent and coercive [...] [the] Black Guerrilla Family (BGF); and a Caucasian gang — the Aryan Brothers (AB).
[US]K. Scott Monster (1994) 343: Next to you is an A.B. and on the other side is an E.M.E.
[US]G. Pelecanos Night Gardener 37: People say they saw the guards passin out knives to the A.B.s, right before they cornered Red out in the yard.

4. attrib. use of sense 3.

[US]S. Morgan Homeboy 125: The A.B. General didn’t have to stand in the line with the others.

5. (Aus. prison) Aryan Breed; the Aus. version of sense 3.

[Aus]M.B. ‘Chopper’ Read Chopper From The Inside 124: There is a group called the AB, taken from an American idea. It stands for Aryan Breed or Aryan Brotherhood.