/omedetaku/ → /omedetau/ → /omedetɔː/ → /omedetoː/
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.
Most often written in hiragana. May occasionally be seen spelled in kanji, generally for more formal writing. The kanji spellings are examples of ateji. Usually followed by ございます (gozaimasu, “it is”, formal) in less casual contexts.
- 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