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/
- 1988, 国語大辞典（新装版） (Kokugo Dai Jiten, Revised Edition) (in Japanese), Tōkyō: Shogakukan
- 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