A coward, a villain; a person who breaks the law or a code of honour; an outlaw.
Old English; earliest use found in The Anglo-Saxon Chronicle. From early Scandinavian (compare Old Icelandic níðingr, Old Norwegian níðingr (Norwegian niding), Old Swedish nīþinger (Swedish niding), Danish nidding) from the Scandinavian base of Old Icelandic níð + the Scandinavian base of Old Icelandic -ing. Compare Middle High German nīdinc, nīdunc envious person (German (archaic) Neiding envious person). Compare also post-classical Latin nidingus, nithingus.