Green’s Dictionary of Slang

chicken-liver n.

[backform. f. chicken-livered adj.]

a coward.

[UK]R. Macaulay They were Defeated I iv n.p.: Pox on him for a chicken-liver that ran away from the Scots. When I go for a soldier, I shan't run from the enemy.
[US]Monteleone Criminal Sl. (rev. edn).
D. Ward Sting [film script] Butt out, chicken-liver.