white liver

"World anarchist ideas, which is another word for treason or white liver," observed the colonel.

"Ye've a better pull than any white-liver of 'em, from Tom's to Barnegat," gasped Bowlegs, struggling against the surf.

Lilly-liver'd is cowardly; white-blooded and white-liver'd are still in vulgar use.

He was simply calling Failing a milksop and a white-liver; just as plainly as if he had used the words.

What is the origin and explanation of the supposed white liver?