It wasn't. It was a term - frequently sarcastic eventually, as in this cartoon - used to describe the "noble" behaviors of southerners.

Here's an article from the time on it, talking about how the rumored "southern chivalry" was anything but: https://www.historians.org/teaching-and-learning/teaching-resources-for-historians/sixteen-months-to-sumter/newspaper-index/new-haven-daily-palladium/southern-chivalry

As someone else.mentioned, a form of it became "southern hospitality" and survived, usually in the complimentary way.