n. The surreptitious movement of personnel out of an enemy's territory; the theft and/or smuggling of goods out of an enemy's territory.
2001
The MH-53J's mission is to perform low-level, long-range, undetected penetration into denied areas, day or night, in adverse weather, for infiltration, exfiltration and resupply of special operations forces.
1984
Occasionally, of course, the military gets so carried away in its passion to rename things that it cannot persuade anyone to use its most imaginative terms. Resisting any mention of retreat, it devised the word 'exfiltration,' but even its own spokesmen find that hard to say."
1983 (earliest)
Until Stockman's arrest on April 25, 1982, soldiers caught in smuggling attempts were not court-martialed. … According to Sgt. James M. Welker … a more severe punishment was meted out to Stockman and Pierce partly because Army officials in Berlin 'firmly believe that, based on past Soviet reaction, the Soviet Union might attempt another blockade of the city because of acts of exfiltration' by American servicemen.
The first sense of today's term is clearly the opposite of infiltration. The second sense is more problematic. It may simply be an unfortunate generalization of the first (unfortunate, since an infiltration applies only to people, not objects). However, it may also have sprung from the non-military meaning of exfiltration: "The action or process of filtering out." Since both senses are used in military contexts, my vote goes to the generalization theory.