Medetai: 2 Different Meanings of the Japanese Word

With positive meanings, Medetai (めでたい) reminds many Japanese of red sea bream, as in its word, Tai (たい) can mean sea bream or 鯛 (Tai). 

The truth is that Medetai is derived from Mede-Itashi (愛で甚し) and is unrelated to fish.

But Taiyaki pancake got its idea from Medetai, and many people associate the word with a fish.

Meaning of Medetai (めでたい)

Medetai originally means: so good (excellent or splendid), like no choice but to admire. But its contemporary meaning is often: auspicious or happy or good news, like

• めでたい日 (Auspicious day)
• めでたい色 (Happy color)
• それはめでたい (that is Good News)

Omedetai (おめでたい)

In the Japanese language, the prefix お (O) can be an honorific. And you can use Medetai and Omedetai interchangeably, like 

• おめでたい日 (Auspicious day)
• おめでたい色 (Happy color)
• それはおめでたい (that is Good News)

In many cases, we use Omedetai in the same way as Medetai. But the former can have a sarcastic connotation (like naive or easily deceived) with a person, so be cautious.

• 君はおめでたいね (implicitly meaning: you are Naive)
• おめでたい人 (in other words, 世間知らずな人 or Naive person)

(Reference Page: Goo Japanese Dictionary めでたい )