おめでとう

• (Tokyo) おめでとー [òmédétóó] (Heiban – [0])[1]
• IPA^(key): [o̞me̞de̞to̞ː]

/omedetaku/ → /omedetau/ → /omedetɔː/ → /omedetoː/

Originally a compound of 御 (o-, honorific prefix) +‎ 愛でたく (medetaku, “admirably, auspiciously”), from adjective 愛でたい (medetai, “admirable, auspicious”).[2][3][1]

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.

• 御目出度う, 御芽出度う, お目出度う, お芽出度う

おめでとう (rōmaji omedetō, historical hiragana おめでたう)

1. congratulations, congrats

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.[2][3][1]

A pun on the congratulatory meaning of omedetō, using the common kanji spelling of 御目出 for the omede- portion and spelling the final -tō with the character 糖 (tō, “sugar”).[2][3][1]

おめでとう (rōmaji omedetō, historical hiragana おめでたう)

1. 御目出糖: a type of Japanese sweet made from rice and azuki bean paste, eaten at times of celebration

• Japanese_language#Late_Middle_Japanese on Wikipedia.Wikipedia
• Late_Middle_Japanese#Adjectives on Wikipedia.Wikipedia
• ウ音便 ("u" sound shift) on the Japanese Wikipedia.Wikipedia ^ja