I don’t think it started as a derogatory term, but it became so. People would use it to imply that someone was ugly or otherwise defective, so no husband would want them. A man unattached is still seen as more acceptable than a woman unattached even today.
In the same way that any word for an independent woman can be derogatory. It wasn’t used in a negative way when I was growing up (in a deeply old-fashioned part if the UK), and as a kid before I knew that being a lesbian was possible I thought of it as aspirational
I don’t think it started as a derogatory term, but it became so. People would use it to imply that someone was ugly or otherwise defective, so no husband would want them. A man unattached is still seen as more acceptable than a woman unattached even today.
In the same way that any word for an independent woman can be derogatory. It wasn’t used in a negative way when I was growing up (in a deeply old-fashioned part if the UK), and as a kid before I knew that being a lesbian was possible I thought of it as aspirational