A "Knave"; often used in Old English, has several meanings. It can refer to a crafty, dishonest, unprincipled, rude and archaic fellow, a male servant or even a Journeyman. This term was used popularly by Knights of olden times to refer to a person who showed no honor, chivalry or just someone who was not knightly.
This may be personified in this card's effect, as knights always showed honor toward their adversaries. By the same token, any "dishonored" monster that inflicts Battle Damagebounces back to one's hand.