I'm reading a book by Kido, in Japanese "Hanshichi Torimonocho". Evey woman escept one or two has a nam beginning by O (Otaku, Okimi....) Sometimes, Otaku-san, which I understand, bot onc eor two for example "Okimi-don"
What does -don mean ?
As you might probably know, -san is a suffix attached to personal names to express mild respect. Likewise, -don is a dialectal suffix from the Kyūshū area. Kidō may have used it just to show the speaker was from somewhere far away from Edo.
In fact, both suffixes come from different ages. First, tono (noble residence) derived the suffix -dono for the noblest people who could afford large residences (during the Hēan era). This was further contracted to -don, which engenders lesser degree of respect. By the middle ages (Muromachi at the earliest), the respect element in -dono had undergone considerable devaluation and another suffix came into use, -sama. This later derived -san.
Currently, -sama and -san are in active use, with lesser degree of respect for the latter. It is often used as a fit-for-all title out of egalitarian ethos. While -don is rare except in dialects, -dono is used in the most formal documents with legal or administrative purposes. Some people don't like it even in those contexts, but -sama feels too subservient for the gravity of the documents.