The work is still visible in the word even when it describes marriage.
Spin names the action of drawing out and twisting fibres. The occupational formation spinnestre named a woman who spun. Spinster later acquired its marital-status uses.
Spin is shown as the recognizable base rather than a claim about one medieval spelling. The trail does not assert a single settled social cause for the later change.
Sources
- Etymonline: spinster
The entry records Middle English spinnestre as a female spinner, formed with spin and the occupational suffix, before marital-status uses.
- Merriam-Webster: spinster
Definitions and the first-use record distinguish the older occupation from the later unmarried-woman sense.