A military term used for a male-male-female threesome in which the woman is providing oral sex to one man and vaginal or anal sex to the other.
The term is derived from the C-5 Galaxy aircraft, which has a hinged nose in addition to the aft cargo hatch. As such, the aircraft can be loaded from the front and the rear.
For a similar act, see the Eiffel Tower.
We did a C-5 on Jill last night, she wanted to get loaded from the front and back.
Jill was doing a C-5 with Jack and me last week. We loaded her from the front and the rear.