the suffix applied to terms for non-white ethnic groups (i.e. African-American) to indicate that American it's describing is different from other Americans-- because as everyone knows, "American" is an ethnicity. And if one is not WASP, he isn't deserving of the singular appellation, and must have a stupid prefix slapped onto it before it's considered "accurate".
origin: the United States' incompetency when addressing its diversity.
Mr. Smith is a citizen of the US and is of German descent. Therefore he is American.
Mrs. Johnson is a citizen of the US and is of English descent. Therefore she is American.
Mr. Obama is a citizen of the US and is of partial African descent. Therefore he is-- wait, what was that? That one would actually be African-hyphen-American.