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1.
C-5
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.
by Buggermy August 18, 2009
 
2.
c5
A radio transmission in Counter-Strike, meaning:

I'm in position,

but said in normal life
c5, c5!!! wtf?
by Lorne Richardson July 06, 2003
 
3.
C5
Lingo short for "class 5" used by Canadian Tire staff to signal to each other of a hot female shopper.

There are different types of C5's, for instance:

C5-code L = Latina
C5-code M = MILF
C5-code LG = Little girl
C5-code P = Pregnant
C4 = Not as hot as C5 but still worthy to point out
C3 = Decent
C2 = Put a bag over that shit!
C1 = Why are you even pointing her out?
1. "Yo Chris check out that C5 code LG down aisle 3!"

"Dude, way too young!"

"Give her a few years, she has potential"

2. "C5 aisle 9"

"I'd say she's more of a C3, but I'd still bang her."
by GCSRT8 September 24, 2011