More can be written, especially because the previous discussions from the FAQ are getting, ahem, long in the tooth. But you might like to read "What did people use before the invention of the toothbrush, floss and scissors?" with answers by /u/Aerandir , /u/speculativereply , and /u/adso_of_melk .

This is not to discourage discussion. More questions, data, and debate are welcome.

The first historical mention of modern type mouth washing is Odol, patented in 1892 by Karl August Lingner in Dresden, Saxony, German Empire.It was developed by the chemist Richard Seifert, who also co-invented artificial leather and Acetylin (a few years before Bayer's Aspirin).If you ever happen to visit Dresden, you may find the German Hygiene Museum interesting. It has a big exhibition on mouth wash and other products of hygiene, especially focussing on how general population became aware of everyday healthcare and grooming.

Oh, and yes, it was founded by Mr. Lingner, for purely educational purposes of course! ;)