Cowardly.

I suppose it is the job of idioms to provide richness to the language by creating meaning that is different to the literal meaning of the idiom’s individual words. Almost any idiom serves as an example – ‘cloud cuckoo land’, ‘fancy-free’, ‘hat trick’ and so on, but ‘lily-livered’ must seem especially opaque to non-English speakers endeavouring to learn the language. Why would that mean cowardly?

One clue is that our Middle Ages predecessors believed the liver to be in control of our emotions. It was thought to be the organ that created blood and that a poorly functioning liver was the cause of mental or physical weakness. Anyone who was choleric, bilious or irritable was labelled ‘liverish’. There were numerous ‘livery’ conditions:

liver-hearted, or lily-livered – craven, cowardly
liver-faced – mean spirited
liver-lipped – pale and feeble
liver-sick – suffering from dropsy, or the diseases we now call cirrhosis and hepatitis.