:As noted above, it's offensive to people of Japanese descent living in America, Canada, and other countries.  During WW2 there was a lot of anti-Japanese propaganda, and anti-Japanese racism.  There were large J-A communities, and they suffered racism.  The term "jap" was used on signs reading "No Japs Allowed", and in wartime headlines. 

 :The anti-Japanese racism existed long before WW2 - the article is incorrect by the way.  Anti-Asian racism existed in the US first against Chinese who came over during the California Gold Rush.  Then, when Chinese immigration was stopped, Japanese immigration increased.  Anti-Japanese anti-immigrant sentiment grew.  Some cities had a "no japs" policy. 

 :Perhaps, back then, "jap" was simply considered an abbreviation - but it was an abbreviation that was associated with racists.  Imagine if anti-illegal-immigrant activists started calling Mexicans "mexies" - that term would start to be considered racist.  Usage defines the real definition of the word. 

 :"jap" was made a slur by 60 years of legalized discrimination and hatred.  Finally, it's being undone, and perhaps one day, "jap" will be more comical than hateful, but we're not there yet.  Old folks who suffered the slur are still alive.  It was alive and well as hate speech in the 70s and 80s (when there was a lot of Japan bashing).  Perhaps someone who is 30 is at the leading edge of getting beyond "jap", and "jap" will join the ranks of "heeb", "kike", "wop", "dago", "mick", "jerry", and slurs I don't even know.