I seem to recall from somewhere (L&T?) that the contraction "an't" was viewed very much like today's "ain't": a sign of low breeding. And in S&S, I had always associated this word with the Steele sisters.

So I was astonished to discover that the first usage of this contraction comes, in chapter 34, from ... Fanny Dashwood! "They are very pretty, ma'am -- an't they?"