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Dental floss has a history that his maybe longer than of the toothbrush but we know for sure that it is in use since the 19th century. It is in the wider use since the invention of synthetic materials like nylon.
Since ancient times we had problems with crooked teeth. They interfered with proper eating and were not too aesthetically pleasing. Because of that, we tried to invent mechanical aids that helped us to fix them. Read about history of orthodontics.
Dental instruments progressed for nearly 9000 years and from very primitive tools they developed into sophisticated equipment that can still seam scary but not so much when you compare them with tools people fixed their teeth some 100 years ago.
Since the ancient times, we try to keep our mouth and teeth clean. Until we found out how bacteria work, we used some pretty unsanitary methods for that but in time we learned and today our hygiene standards are pretty high. Read more about history of oral hygiene.