a freelance reporter or local correspondent who gets paid by the story. Origin: one theory dates back to the time when part-time reporters were paid for each column inch of copy the newspaper printed. At the end of the week/month, they would measure the space their writing occupied in the newspaper by a length of string. They'd turn in the piece of string, and would be paid according to how long the string was.
The Times stringer in Podunk was the first to break the big story.