When I was in high school a wigger was just a classification, kind of like a stoner, skater, nerd, or jock. Just about everyone fit into some category. We didn't psychoanalyze the word. Now, if they actually said white ****** it has different racial undertones but it is still not worth getting into a fight over.
It's racist.
Wigger stands for "white ******." It's refused to refer to white kids who "act black," instead of "acting white." The very fact that people had a name for kids who "act black" is racist.
Just because you didn't understand that it's racist, doesn't mean that it isn't racist.
You might want to check your privilege sometime.