|from the Palos Verdes News volume XIV|
Jap was used as a pejorative during World War II. It was used interchangablely for the people of Japan who America was at war with, as well as Japanese-Americans (many of whom were removed from the west coast and interned or served in the armed forces). According to both the Oxford and Merriam-Webster dictionaries, it is considered a racial slur. It is not the equivalent of Brit or Aussie. Unless you intend to come off as a racist, do not use this slur.
This link is to a discussion of the twitter phenomenon and here is a link to the Densho: The Japanese American Internment where you can learn more about the treatment of Japanese Americans during World War II.