Frontier comes from Old French frontiere meaning 'front line', from Latin frōns meaning 'forehead'. A frontier is the forehead of a territory — the face it turns toward the unknown.
A line or border separating two countries; the extreme limit of settled land; the boundary of knowledge or achievement.
From Old French frontiere meaning 'front line, boundary, border', from front meaning 'forehead, front', from Latin frōns (genitive frontis) meaning 'forehead, brow, front'. A frontier is, literally, the forehead of a territory — the part that faces outward toward the unknown. The metaphor is anatomical: just as the forehead is the forward-facing part of the head, the frontier is the forward-facing edge of a country
A frontier is a forehead. Latin frōns meant 'brow', and the frontier was the brow of a nation — the part facing outward. The same root produced confront (to stand forehead to forehead), affront (to strike someone in the face), and effrontery (bare-faced boldness, literally 'removing the forehead' of shame).