He has a white feather; he is a coward; an allusion to a game cock, where having a white leather is a proof he is not of the true game breed.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
White Ribbin * White-liveredNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes