Its first use was in 1820 in the book Ivanhoe to describe a medieval mercenary. The warrior's lance was "free" of any obligation to a certain lord or kingdom, but was up for hire. Of course, the meaning has changed significantly since then.
And if you don't mind a bit of vulgarity and innuendo,
A Knight's Tale is certainly a good movie for a laugh.
So, what does freelance have to do with writing?
Freelance writers may write short articles for newspapers, blogs, or other publications, changing up where they work based on what jobs are available. Freelance editors are in business for themselves (rather than working for a publishing company or some such) and do various types of work for different authors and publications.
Have you ever considered freelance writing or editing?