Some of the negative connotations of the word spinster evolved in the 19th century, when spinning-houses were used as holding cells. Women of a certain profession found out at night after a certain hour (It must be dirty- it can only be referred to in vague terminology.) were given the more productive labor of spinning flax and wool instead of being out performing lewd and lascivious acts. They were referred to as spinsters (and, incidentally, this term could also apply to men) Thus a spinster began to be associated with people of loose moral character. Interesting the way meanings change, eh?