It’s quite a bit older than that. 1800s, I believe? A “spinster” was a women who got a job spinning/weaving fabric, which was one of the few jobs women were allowed at the time, and they were paid for it! Very well, iirc.

So a woman earning her own income and able to support herself, not being required to marry in order to live comfortably.

Yes, and well before that too. It meant an unmarried adult woman over the age of _____. (Here is where the discrepancy lies.) It was always true for an elderly woman. But could sometimes be applied all the way down to age 30, especially if you go far enough back that you were expected to be married in your 20s. (And if you weren’t, there must be something wrong with you.)