Military slang for "the person who takes charge and gets shit done."
This person is not necessarily the highest ranking in the group.
You come upon a bunch of people standing around a vehicle with a flat tire, smoking and joking. You notice there is a leadership vacuum - no one is taking charge, so you ask, "Who is fucking the chicken here? Where's the chicken fucker?" in such a way that inspires the most ambitious and motivated person to step forward into a strong leadership role.