いお【硫黄】 : io | define meaning

Cognate to Kazuno (Akita), Chiba and Shizuoka ゆおう yuou; Chiba ゆを yuwo and よー yoo; Kagoshima いお io; Okinawan ゆーわー yuuwaa; Miyako よー yoo; and standard Japanese いおう・ゆおう【硫黄】 iou ~ yuou "sulfur, sulphur". Spelled in classical Japanese as ゆわう yuwau and いわう iwau. Possibly also related to Mandarin 【硫黄】 liúhuáng, Cantonese lau4 wong4, South Korean 유황 yuhwang, North Korean 류황 (r)yuhwang, and Min Nan 【硫磺】 liû-hông "sulfur, sulphur".

The form 【由乃阿和】 *yu no awa (modern ゆのあわ【湯の泡】), literally "hot-water bubbles", occurs in the Wamyō ruijushō (倭名類聚抄), which may suggest the term later became ゆあわ【湯泡】 yuawa and finally ゆわう yuwau "sulfur, sulphur" in Old Japanese. However, the Wamyō ruijushō also contains the form 【由王】 *yuwan or *yuwau, which may suggest the term came from Chinese 【硫黄】 (cf. Mandarin liúhuáng). Two things cast doubt upon this theory: 1) the *h quality in the second syllable would have resulted in *k in Japanese (counter-argument: 卵黄 ran'ou), and 2) the initial *liu should have resulted in *riyu and later *ryuu in Japanese.