The term "wigga" is a combination of the word white and the racial slur "nigger." Which has evolved to "nigga" with the common reference from African-American to African-American. The word basically refers to Caucasian males who have a psychological mindset, which makes them think that they are African Americans. They typically dress in hip-hop clothing (2 to 3 times their size), which they refer to as "urban wear". These people are typically wealthy Caucasian males, which live in the suburbs and do not have any affiliation or any idea of what the ghetto is like.
White (Caucasian)+Nigga (Nigger)= WIGGA
Eminem is considered to some African Americans as a "wigga". Although some accept him because of his lyrical talent.