means A remote settlement, military station, or branch of an organization positioned far from the main center, often marking the edge of controlled or familiar territory.

from A transparent compound of "out" and "post" — "post" here in its military sense of an assigned station or position, from Latin "positus," "placed" (the same root behind "position" and "posit"). So an outpost is literally a station placed out beyond the main body, a guard set forward at the margin. The word grew up in military use, where a small detachment was "posted out" ahead of an army to watch for trouble, then spread to mean any far-flung foothold.