According to Paul Fussel's book "Wartime," "Chickenshit refers to behavior that makes military life worse than it need be: petty harassment of the weak by the strong; open scrimmage for power and authority and prestige; sadism thinly disguised as necessary discipline; a constant 'paying off of old scores'; and insistence on the letter rather than the spirit of ordinances. Chickenshit is so called--instead of horse- or bull- or elephant shit--because it is small-minded and ignoble and takes the trivial seriously."
This definition is cited by at least two books by Steven E. Ambrose, "Citizen Soldiers" and "Band of Brothers."
Soldier 1: I got fined $25 this morning!
Soldier 2: What for?
Soldier 1: Some rear-echelon type saw me and said that I wasn't maintaing a professional appearance. I can't help it that we've been living in foxholes for the last week and because of the snipers, I can't shave.