I believe that the name of the nation in Japanese transliterates to Nippon in English.

Nippon is the English equilivent for the word in Japanese that actually means Japan... Nihon.

Because the Japanese call their country Nippon in their own language. The word has somehow got recognized in English too. Hence the term "Nipponese".

1) "Etymology From Nippon (romanized version of 日本, the native Japanese name for Japan) + -ese." Source and further information: http://en.wiktionary.org/wiki/Nipponese 2) "Neal Stephenson aside, the only period when "Nipponese" was used widely was World War II, when Japan was officially known as the Dai Nippon Teikoku or "Great Nipponese Empire," and always referred to itself as such. "Nipponese" was chosen over "Nihonese" or "Japanese" because it sounded tougher (I am not making this up). Of course, that ended pretty quickly when American GI's got a hold of the word and transformed it into "Nip," the preferred derogatory slang for the Japanese during the war. After the war, the Japanese mysteriously stopped using "Nippon" in English: now you mostly find it in abbreviations like ANA, NTT, and NHK. The official name of Japan today is Nippon Koku, "State of Nippon," and the word "Nippon" appears in romaji on its postage stamps. Nihon is the preferred Japanese term for referring to Japan without specifically referring to the government. Only the worst of Japanology's pedants continue to use "Nipponese" in conversation or writing, but they're out there." Source and further information: http://everything2.com/index.pl?node_id=89499&lastnode_id=0

Um.... they aren't. People of japan are known as "Nippon-jin" or "nihon-jin". or "nihon-go" when referring to a Thing being Japanese. but "-jin" is for people. nipponese isn't a real word. "Nippon" is a japanese word. "Japan" and "Japanese" are english words, so "-ese" is english, not japanese.