alibi

(noun) (law) a defense by an accused person purporting to show that he or she could not have committed the crime in question.

(noun) a defense of some offensive behavior or some failure to keep a promise etc.. Synonyms: excuse, exculpation, self-justification. Examples: "He kept finding excuses to stay." "Every day he had a new alibi for not getting a job." "His transparent self-justification was unacceptable."

(verb) exonerate by means of an alibi.