Answer :
A frontier is an area of land that is near or beyond a boundary, typically the land that comes after the boundary. Back in the early to mid 1800's the philosophical idea of 'Manifest Destiny" was born. Basically America was born to expand so they could spread ideas like democracy and freedom. As the 1800's arrived, American settlers moved west, trying to expand their nation. They conquered a large portion of Nowadays US, through the Louisiana Purchase, the Mexican-American war, and many resolutions with Native Americans. The only place that was yet to be conquered was the Oregon country otherwise known as the Pacific Northwest. Much of the US was owned by various countries but were given to America by war, US expansion, or peace treaties. Great Britain owned the territory and didn't want to give it up, but President Polk threatened to wage war, and considering how US has won many recent wars, UK didn't want that. Eventually the US received the now-day Pacific Northwest territory (Oregon, Washington) as a settlement over the land and boundary dispute. Acquiring this land shaped the US country we now know as the final boundary was set between the US and present Canada hence the idea of final frontier.