His informal language suggests that he might not really be an upper-class duke.
We know that the duke does not actually own any of the houses since he is a traveling con-artist. This also proves that he is not really an upper-class duke. The use of the word "shin" does not show his knowledge of human anatomy since this was a common phrase used to tell someone to run fast to avoid trouble. The "duke" does use plenty of informal language such as "till", "shin", and "dickens". This does suggest he is not actually an upper class duke.