Green’s Dictionary of Slang

chickenheart n.

a coward.

[1784		Rambler’s Mag. June 239/1: Mrs Cuyler was lately seen [...] driving her gig along Hyde-park — the beaus looked, sighed, and their chicken hearts beat with tumultuous palpitations; but the laduies were of the imperative mood].
1845		J. Lindridge Sixteen-String Jack 114: Nonsense, chicken-heart.
1910		O. Johnson Varmint 85: ‘You quitter!’ ‘You chickenheart!’.
1949		(con. 1943–5) A. Murphy To Hell and Back (1950) 16: Mike Novak is not to be soft, no chicken heart.
1960		D. Hamilton Death of a Citizen 63: Washington is the city of the soft heads and the chicken hearts.