Unlike singular "their", the word "ain't" never
appears in the narration of Jane Austen's novels, but is confined to the
quoted dialogue of certain characters of dubious elegance and
correctness. It is interesting that "ain't" mainly occurs in her
first-published novel Sense and Sensibility (except for two
occurrences of the phrase "Ain't I?", which might be interpreted as
intentionally humorous, rather than as reflecting on the characters' bad
grammar).
Occurrences of vulgar "ain't" in Sense and Sensibility
Nancy Steele:
"Nay, my dear, I'm sure I don't pretend to say that there an't. I'm
sure there's a vast many smart beaux in Exeter"
Mrs. Jennings:
"Well, my dear, 'tis a true saying about an ill-wind, for it will be
all the better for Colonel Brandon. He will have her at last; ay, that he
will. Mind me, now, if they an't married by Midsummer. Lord! how he'll
chuckle over this news!"
Fanny ("Mrs. John") Dashwood:
Perhaps Fanny thought for a moment that her mother had been quite
rude enough; for, colouring a little, she immediately said, --
"They are very pretty, ma'am -- an't they?" But then again the
dread of having been too civil, too encouraging herself, probably came
over her...
Lucy Steele:
"Are you ill, Miss Dashwood? -- you seem low -- you don't speak; --
sure you an't well."
Nancy Steele (to Elinor):
"Edward talks of going to Oxford, soon," said she; "but now he is
lodging at No. ----, Pall Mall. What an ill-natured woman his mother
is, an't she? And your brother and sister were not very kind!
However, I shan't say anything against them to you..."
Mrs. Jennings (to Elinor):
"Lord! my dear, you are very modest. I an't the least astonished
at it in the world; for I have often thought, of late, there was
nothing more likely to happen."
Mrs Jennings:
"The Colonel is a ninny, my dear; because he has two thousand a
year himself, he thinks that nobody else can marry on less. Take my
word for it, that, if I am alive, I shall be paying a visit at
Delaford Parsonage before Michaelmas; and I am sure I shan't go if
Lucy an't there."
Occurrences of first-person jocular interrogative "ain't" in the other novels
Charles Musgrove:
"Well, mother, I have done something for you that you will like. I have
been to the theatre, and secured a box for to-morrow night. A'n't I a good
boy? I know you love a play; and there is room for us all."