Game concept
Grudge
The game also links it as Grudges, Grudge's, Grudges'.
From the game's encyclopedia
A Grudge is a Special Relation indicating unilateral hatred and resentment. The target of a grudge does not return the feelings and may not even know.
Grudges are most commonly formed when one Character does something horrible to another without even realizing they exist — for instance, a king leaving a peasant in their Dungeon for forty years.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Grudge is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Grudge is a Special Relation indicating unilateral hatred and resentment." It is part of Special Relation. The game also links it as Grudges, Grudge's and Grudges'. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.