Righteous anger stirred by something felt to be unjust, base, or shameful.
Righteous anger stirred by something felt to be unjust, base, or shameful — moral anger at a perceived wrong.
Moral anger — the feature it shares with outrage and which separates it from anger, ire, fury, rage, exasperation, vexation, and pique (which need no moral trigger). Its trigger is a perceived injustice or violation of fairness/dignity, with a self-righteous coloring. Less intense and more contained than outrage: the controlled, principled anger of someone affronted by wrongness, often felt on others' behalf — whereas umbrage and resentment center on a slight to oneself.
c. 1200, from Latin indignationem “displeasure, indignation,” from indignari “to regard as unworthy, be displeased at,” from indignus “unworthy” (in- “not” + dignus “worthy”).
The “unworthy / righteous-anger” core has been stable since c. 1200; the moral dimension is original, not a later development. No reliable recent-generation shift is sourced.