Definition of final frontier in US English:

final frontier

noun

  • 1A part of a country, region, etc. that is the last to be settled, explored, or developed.

    ‘the final frontier was Antarctica’
    1. 1.1 The extreme limit of understanding or achievement in a particular area.
      ‘the artist stands on the final frontier of the hip-hop universe’

Origin

Mid 19th century: popularized by its use in the opening of the US television series Star Trek.