1757, "military position detached from the main body of troops or outside the limits of a camp," from out- + post (n.2). Originally in George Washington's letters. Phrase outpost of Empire (by 1895) "remotest territory of an empire" in later use often echoes Kipling:
There he shall blaze a nation’s ways with hatchet and with brand,
Till on his last-won wilderness an Empire’s outposts stand!