A wonderful breakdown and etymology of the word chickenshit by the author Paul Fussell from his book Wartime, a second lieutenant with the 103rd Infantry Division in WWII. Found and quoted from the book Citizen Soldiers by Stephen E. Ambrose.
The guys who were permanent jerks were the usual suspects — officers with too much authority and too few brains, sergeants who had more than a touch of sadist in their characters, far too many quartermasters, some MPs. The types were many in number and widely varied in how they acted out their role, but the GIs had a single word that applied to every one of them: chickenshit.
Fussell defines the term precisely. “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… 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. Chickenshit can be recognized instantly because it never has anything to do with winning the war.”