Combat Jack is generally known amongst military personal as masturbating during wartime. It was the name of an episode of Generation Kill, in which we see various soldiers in the platoon "relieving" themselves in humvees or while sleeping.
Other slang terms- Rub one out, jack off
From Generation Kill--
Soldier- Where did you go?
Reporter Evan Wright- I dont know..i was trying to find a secluded place..
Soldier- Combat Jack??
Reporter Evan Wright- No...i was trying to take a shit and ended up by the front gate, then everyone started shooting.
To jack off in the middle of a firefight, or any combat situation.
Bob: Oh shit man, I'm scared of these Sammies.
Jim: Let's just combat jack to relieve the anxiety!