[ralferoo]

-(e)th was the Middle English ending for 3rd person singular and plural present tense. It has mostly been replaced by -s in Modern English, except for a few fixed expressions (e.g. my mum used to say "quoth he" which is subtly different from the modern "he quoted" or "he quotes" which is normally followed by an object that is being quoted as well as the quote itself).

Middle English is quite interesting in general, as it still has a pretty regular inflection system for verbs that's largely been lost now (3rd person singular -s is the main vestige): https://en.wiktionary.org/wiki/Appendix:Middle_English_verbs...

It's also interesting to look at the case system for ME nouns too, as it makes some things easier to understand. For instance the 's in Modern English for possessive is really just an abbreviation for the -es genitive case, which probably occurred when spoken forms had changed from -es to -s, which in turn was probably due to the shift to drop terminal -e from words which had started even by Chaucer's day (some places in Canterbury Tales you need to treat the -e as its own syllable to hear the rhyme, in other places it needs to be silent).