While recently reviewing the keywords people use that bring them to my site, I came across this question:
what is the proper noun of happy
To answer my initial interpretation of the question, the only noun form of “happy” I know of is “happiness”. I guess, by default, happiness is the proper noun of happy.
That being said, the search results point to an old post I wrote on how to properly wish someone a happy birthday, specifically regarding capitalization. So, perhaps the person who typed that question into Google was trying to figure out if “happy” should be capitalized (proper nouns are capitalized) even though “happy” is an adjective.
Again, the noun form of happy is happiness. And only capitalize “happy” in a greeting if it is at the start of a sentence.
Hope that answers the question.