is also the person in the tail of a combat aricraft, usually a heavy bomber
such as the b-17
or the more modern b-52
who augments defensive capability for the aircraft
, namely preventing the plane
from being attacked from the rear. This isn't much use unless the attacking plane is coming from the rear to use his guns. This is required because the very heavy bombers cannot out-manouver the smaller and much more agile fighter aircraft.
This is also a euphemism for a male homosexual, think for a second about the stereotype and I'm sure that you'll see how it could be applied. ( this excersise is left for the reader)