"... in favour of Southern womanhood as much as anybody, but not for preserving polite fiction at the expense of human life."
This particular phrase that Atticus says to Aunt Alexandra shows Atticus' use of formal language and clear stance on the Tom Robinson issue. This phrase also helps understand Scout's immaturity from her lack of reaction to this. Atticus feels strongly about Tom Robinson losing his life at the expense of "preserving polite fiction."