Can be a number of things. Mostly it's the new flavor of the week, they are called this til they graduate to "Baby Momma" or "The Wife." Often times, old girl is a called this because she is kept on the DL and/or your ashamed of her.
"I think old girl put something in my drink"
Reggie: Whatchu doin tonight?
Ab: Chillin out with old girl.
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