knave (n.) Look up knave at Dictionary.com
Old English cnafa "boy, male servant," common Germanic (cognates: Old High German knabo "boy, youth, servant," German knabe "boy, lad," also probably related to Old English cnapa "boy, youth, servant," Old Norse knapi "servant boy," Dutch knaap "a youth, servant," Middle High German knappe "a young squire," German Knappe "squire, shield-bearer"). The original meaning might have been "stick, piece of wood" [Klein]. Sense of "rogue, rascal" first recorded c.1200. In playing cards, "the jack," 1560s.