If I remember correctly, much of Tom Sawyer is written with a pseudo-phonetic spelling of very informal American-English used by the characters in the book. Sometimes if you read it out loud and try to imagine the accents of the characters, it can help. Even for native speakers of American English it's still difficult to understand, but the book is charming and worth the trouble.
In isolation, one might think that but it's more likely dialect than pronunciation. Aunt Polly's next utternances are:
“Well, I lay if I get hold of you I'll--”
“I never did see the beat of that boy!”
This is soon followed by an instance of negative concordance and we're still on the first page.