look up any word, like plopping:
 
1.
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.
by Manny Diaz May 01, 2005
 
2.
Your mom. Mother
my old girl asked me to pick her up somthin at da store.
by LiL Neice April 07, 2003
 
3.
As above.
Very common in Australia.
by Diego November 05, 2003