Green’s Dictionary of Slang

(orig. US) a coward; a contemptible, disgusting person.

1945		‘GI Lingo’ in AS XX:2 Apr. 147/2: chicken or c.s. (chicken shit) A person is ‘chicken’ when he abides too closely by army rules and regulations, or when he misuses or abuses authority, expecially in minor or petty matters.
1951		Amer Journal Sociology LI 420: In this case the authority exceeds being GI, it becomes ‘chicken s–,’ sometimes abbreviated to ‘CS’ or ‘chicken’ [HDAS].
1971		I. Faust Willy Remembers 211: C.S. Tom. The lieutenant. The captain.
1973		Lucas, Katz & Huyck Amer. Graffiti 54: C.S. [...] Chickenshit [HDAS].
1989		H. Rawson Dict. of Invective (1991) 80: This last [chickenshit] is sometimes euphemized as C.S., chicken stuffing, or just plain chicken.