[thaumasiotes]

But I didn't ask about how we know the pronunciation. I asked about how we know how to say hello. There are any number of good reasons the word for olive oil might be attested in Greek. It's not so obvious why "hello" would be attested -- it's usually something you say, not something you write down.

[MonkeyClub]

Hello,

A good source would be epistolography, another would be narration of encounters.

[thaumasiotes]

> A good source would be epistolography

This one seems tricky. English letters do not normally begin with "hello", or even with any synonymous phrase. Instead, they use forms that are specific to written letters.