Green’s Dictionary of Slang

about that adj.

(US campus) in agreement with.

1997–2001		Online Sl. Dict. 🌐 about that adj. 1. in support of. Also about it. Note: often pronounced ‘bout that’ or ‘bout it’. (‘I’m so not about that.’ ‘You want to leave now? I’m about it.’ ‘Whatever you want to do, I’m about it.’).
1998		Eble Campus Sl. Apr.

In phrases

about that life

(US campus) in agreement with.

2014

Eble UNC-CH Campus Sl. Spring 2014 1: ABOUT THAT LIFE -- in accord with a particular way of living: ‘I just dropped a couple hundred on the new J’s [Michael Jordan sneakers] . You know, I’m about that life.’.