When Atticus tells Scout not to use the "n" word because it's "common," what does he mean?

Atticus infers that no well brought up child (or adult) would use that word in conversation.... it makes her sound as if she hasn't been raised to know better.

“Do you defend niggers, Atticus?” I asked him that evening.
“Of course I do. Don’t say nigger, Scout. That’s common.”
“‘s what everybody at school says.”
“From now on it’ll be everybody less one—”