Why is the color yellow associated with cowardice?

O Matt:

Today I was watching Gunsmoke, and one cowboy called another "yellow," alluding to cowardice. Why "yellow"? Is this some sort of dis against Asians, or maybe people with liver disease?

-- Jaundiced Joe in email-land

The Old West wasn't very PC, but if my trusty word-origin sources are working at their usual hallucinatory best, the yellow in question is not skin color. As we expected, our sources are again mumbling and clearing their throats and acting as if we're cluttering up their offices with our unworthy questions -- a nifty diversionary tactic when they really haven't a clue about the answer. Anyway, yellow's never had a very good rep. The color of traitors or adulterers. Bad, bad folks. But cowards are probably more closely linked to amphibians. Frogs, specifically. Yellow-bellies. Things that hop away and hide when you go after them. "Yellow" is shorthand for "yellow-belly." At least that's the best I could get out of our daffy word-origin panel. I know I keep saying it, but we really do have to get those fools off the payroll.