This is bad. So please stop.

Tanitha: I dont believe Jap to be offensive, its short for Japanese. Nothing more or less.
Like calling an Australian a Ozzie or Aussie or from AUS or from Oz.
Any word can be used in a derogatory manner, like calling someone a monkey based on their race Etc..
Its not the word but the meaning that makes a lot of things offensive.
I dont believe there is anything derogatory or offensive intended simply by cutting a long 8 letter word down to 3 letters.
However. Looks like the dictionary's disagree with me.
http://www.thefreedictionary.com/Japs
Regards
Tan.