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. Chickenshit can be recognized instantly because it never has anything to do with winning the war.

Paul Fussell (1924-2012) American cultural and literary historian, author, academic

“Chickenshit: An Anatomy,” Wartime: Understanding and Behavior in the Second World War (1989)
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Sometimes misattributed to historian Stephen Ambrose, who used part of it (with attribution) in his Band of Brothers (1992).