Does it encompass any specific age group? (young, middle-aged, elderly, all of them)
I heard it in an old film and read it once in sentences like:
"Come on, old girl, cheer up."
"Who is that old girl you were talking to?"
old girl – a familiar name used to refer to a woman
EDIT – So as to avoid multiple interpretations of the idiom, and too many answers, let's keep this question to its use in BrE.