E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
means your mercy is craved. It was usual in former times to decide controversies by an appeal to battle. The combatants fought with batons, and if the accused could either kill his adversary or maintain the fight till sundown, he was acquitted. If he wished to call off, he cried out Craven! and was held infamous, while the defendant was advanced to honour. (Blackstone.)