Modern Japanese -i adjectives formerly ended in -ki for the attributive form. This medial /k/ dropped out during the Muromachi period, both for the attributive form (-ki becoming -i) and for the adverbial form (-ku becoming -u). However, the adverbial form reverted back to -ku thereafter for most words, with the -u ending persisting in certain everyday set expressions, such as arigatō, ohayō, or omedetō, and in hyper-formal speech.
/ohayaku/ → /ohayau/ → /ohayoː/
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- 1995, 大辞泉 (Daijisen) (in Japanese), Tōkyō: Shogakukan, ISBN 4-09-501211-0
- 2006, 大辞林 (Daijirin), Third Edition (in Japanese), Tōkyō: Sanseidō, ISBN 4-385-13905-9