The word is an old-fashioned seldom used today) for a woman who has never married; when it was commonly used, it carried a negative connotation, since women back then were expected to have a husband. The term was derived from the word for a woman who was a spinner of wool (spinning and weaving were activities that were considered part of a woman's home duties, but some unmarried women also made a living this way). Calling a woman a spinster meant she was considered too old, outside the "normal" age for marriage.
1.(n.) A woman of evil life and character; -- so called from being forced to spin in a house of correction. 2.(n.) An unmarried or single woman; -- used in legal proceedings as a title, or addition to the surname. 3.(n.) A man who spins. 4.(n.) A woman who spins, or whose occupation is to spin.