Green’s Dictionary of Slang

(US black) to wear one’s trousers hanging low, exposing the top of one’s underwear.

1991		L. Bing Do or Die (1992) 243: Flaggin’. Saggin’. Braggin’. Lettin’ people know you’re part of something that is powerful.
1998		Source Oct. 90: We gon’ sag, wear braids, and dress how we wanna dress.
2007		UGK ‘Trill Niggaz Don’t Die’ 🎵 I was sagging in my khaki’s [...] Gangsta Nike’s on my feet.
2012		‘Dutch’ ? (Pronounced Que) [ebook] Adjusting his sag as he slow-bopped to the car.
2016		Jeezy ‘In the Air’ 🎵 I hopped up out that g-wagon / Glock got my Balmain jeans sagging.