It's an ejector and was fitted on the DC-8's equipped with JT3C, JT4A or RR Conway engines. Normally extended for T/O and landing, it is retracted in flight after T/O because it is no longer needed and creates drag at higher speeds, and then it is extended during the descent/approach phase. It is operated hydraulically to extend and retract, and can be extended by an air bottle in the event of a hydraulic failure. The ejector does two things. It reduces noise and improves thrust output up to approx. 150 knots. It also carries the thrust reverser which operates pneumatically to increase descent rate or slow the aircraft inflight and on landing to reduce the landing roll and reduce wear on the wheel brakes. I know this because I was a DC-8 Captain and flew them for 10+ years.