Douglas Harper's Etymology Dictionary
1650s, from self- + justification.
Wiktionary
n. justification of one's self.
WordNet
n. a defense of some offensive behavior or some failure to keep a promise etc.; "he kept finding excuses to stay"; "every day he had a new alibi for not getting a job"; "his transparent self-justification was unacceptable" [syn: excuse, alibi, exculpation]
Wikipedia
Self-justification describes how, when a person encounters cognitive dissonance, or a situation in which a person's behavior is inconsistent with their beliefs, that person tends to justify the behavior and deny any negative feedback associated with the behavior.
Usage examples of "self-justification".
Behind its facade of indignation and righteousness, the line she had forced herself to listen to was, like so many philosophies that she had heard from other misfits and self-styled iconoclasts, really nothing more than a massive exercise in self-justification.