According to the Oxford Advanced Learner's Dictionary of Current English a frontier is a 'part of a country bordering on another country'. In the American English this word has one more meaning: in the past a frontier was the 'farthest part of a country to which settlement has spread, beyond which there is wild or unsettled land' (352). So the language of the frontier is first of all the language of people who settled at the western frontier of United States. The settlers were plain people, backwoodsmen, most of them were illiterate and spoke a simple, substandard English, often even dialects that originated from their old fatherland (Sorbonne). That all influenced of course the language as the whole.
The language of the frontiersmen is well documented and can be an object of scientific studies thanks to tall tales, which were oral stories, before they were written down. So The Tall Tales of Davy Crockett can be a very rich source of examples of words and grammar rules used by people those days. It is also interesting to examine the characteristics of texts written in the beginnings of the 19th century.