Putting words that were not in use in the Regency Period INTO into the mouths, or thoughts of characters is I think very bad ton

An important aspect of this Rant is the clear difference between spoken conversation, thoughts and descriptive text.

I think characters in the Regency period should NOT use words that either did not exist or culturally were not used in either conversations or thoughts.

However, it is moot whether or not authors can use the same words in descriptive text, with the rider that their appearance will look a little peculiar.