a bit confusing
if you regard the a/c at rest and the air moving past it (at the air speed of the a/c natch), to gain thrust (ie accelerate or just overcome drag to maintain same speed), each 'parcel' of air ingested must be accelerated, so it leaves the exhaust at a speed higher than the airspeed. So in the moving a/c reference, the exhaust has a rearwards velocity relative to the undisturbed air.