What is a "Shimanchu"? Okinawa's word for its own people
Why do Okinawans call themselves shimanchu, while people from the mainland are yamatunchu? The answer runs back to the days of the Ryukyu Kingdom.
"Shimanchu" is a warm word for someone born and raised in Okinawa.
Here is how it relates to "uchinanchu" and its opposite, "yamatunchu."
What "Shimanchu" means
Shimanchushimanchu
Written 島人 ("island person"), it means a person of Okinawa — someone who lives there or comes from there. It carries real affection.
Source:コトバンク「島人（シマンチュ）」（デジタル大辞泉ほか）
Its link to "Uchinanchu"
Uchinanchu
"Uchinaa" means Okinawa and "nchu" means person, so "uchinanchu" is a person of Okinawa — used almost interchangeably with shimanchu. A similarly built word is "uminchu," the sea people — that is, fishermen.
Source:沖縄方言辞典 あじまぁ「ウチナーンチュ」
Paired with "Yamatunchu"
Yamatunchu
Okinawa distinguishes its own people (shimanchu / uchinanchu) from mainlanders (yamatunchu). The distinction echoes the days when Okinawa was its own country, the Ryukyu Kingdom. The words are everyday and not unfriendly — simply a way to say where someone is from.
Source:コトバンク「やまとんちゅ」（精選版 日本国語大辞典ほか）
Spelling variants (島んちゅ / しまんちゅ / 島人)
島んちゅ / しまんちゅ / 島人
It appears in mixed kana (島んちゅ), hiragana (しまんちゅ) and kanji (島人). It is often written in kana to keep the sound of uchinaaguchi, the Okinawan language.
Source:沖縄方言辞典 あじまぁ「ウチナーンチュ」
FAQ
Are "Shimanchu" and "Uchinanchu" different?
They mean almost the same thing — an Okinawan. "Shimanchu" is built from "island person," and "uchinanchu" from "Okinawa (Uchinaa) person."
Who is a "Yamatunchu"?
A person from the mainland, as seen from Okinawa. It is the counterpart to shimanchu — not an insult, just a way to say where someone is from.