Dwarfs never forget a wrong, imagined or otherwise, with lords and thanes being particularly overburdened with inherited grudges. These can never be forgotten, only settled or nurtured.
A model with this special rule has the Hatred (enemy characters) special rule, meaning it hates all characters in the opposing army. If this character joins a unit of Longbeards or Hammerers, that unit will also gain this special rule. Should this character leave a unit of Longbeards or Hammerers they have joined for any reason, that unit loses this special rule.