Fare is unrelated to fair:
> From Middle English farewel, from fare wel! (and the variants with the personal pronoun "fare ye well" and "fare you well" used in the Renaissance), an imperative expression, possibly further derived from Old English far wel!, equivalent to fare (“to fare, travel, journey”) + well.*
* https://en.wiktionary.org/wiki/farewell#Etymology
* https://en.wiktionary.org/wiki/fare
So to say "farewell" to someones is "have a good journey (in life?)".