Origins
A country's frontier is its forehead. The word comes from Old French frontiere, meaning 'front line' or 'boundary', from front ('forehead, face'), from Latin frōns — 'brow'. The metaphor maps the human body onto geography: the frontier is the part of a territory that faces forward, toward whatever lies beyond.
In medieval usage, a frontier was a military line — the front of a conflict zone. The defensive sense persists in the idea of frontier fortifications. But the word's meaning expanded as European exploration pushed outward. By the 18th century, the American frontier meant the edge of settled territory, the line between the known and the wild.
Frederick Jackson Turner's 1893 'frontier thesis' argued that American democracy was shaped by the constant existence of a frontier — a moving edge that required self-reliance. When NASA calls space 'the final frontier', it extends Turner's metaphor beyond Earth itself.
Latin Roots
The Latin root frōns produced a rich family. To confront someone is to stand forehead-to-forehead with them. An affront strikes at the face. Effrontery is shamelessness — literally, an absence of the blush that should appear on the brow.
Frontier is now used metaphorically for any boundary of knowledge: the frontiers of science, the frontiers of medicine. In every case, the word preserves its original image — a face turned toward the unknown.