Originally posted by therion
Ok, I apalogize, I´m not native english speaking.
You were also not raised in the United States, so no harm done; you did not know and weren't likely to know.
During World War II in the 1940s when the United States was at war with Japan, "Jap" was used a derogatory word towards the Japanese. It was part of viewing them as the "them" in the "us versus them" mentality that often arises in conflict. It's probably still regarded as derogatory since many people from the 1940s are very much alive, both the people who used the term and the people who were on the receiving end of it.
It'll probably just take time with the changing generations for the word to completely lose its punch. For example, in the LGBT community the older generation considers "queer" to be offensive while many in the younger generation does not.