I have never read Balzac so I may be talking through my hat here but in French Québec, when a woman reaches 30 she becomes a "vielle fille" an old girl.

In French the word fille does not only mean girl. It also means unmarried. When a woman is married she becomes une femme - a woman. The word wife is translated as femme in French.

This is an old expression that fell out of favour in the 70's when it became o.k. *not* to marry.

In my parent's generation (now in their 60's) and before, it was extremely rare for a woman past 30 to marry and she was generally called a vielle fille till the end of her days.

It was not considered an insult - just a name for a particular state of life.

Now, you wouldn't dare call a woman a vielle fille as you probably would get slugged. It morphed into an insulting term, usually meant to mean a woman is so difficult nobody would want to marry her (or if added with a wink, a nudge or wiggling eyebrows) it means the woman is gay.

Phewf, I'll take a breath now . Did ya manage to get all the way through that JJ?

P.S. JJ I don't really know you so I thought I'd add. I am French from Québec, Canada, and when I talk about French I only talk about this place. I really can't speak for the French from France since sometimes they have completely different terms/vocabularies.