alibi (n.) Look up alibi at Dictionary.com
1743, "a plea of having been elsewhere when an action took place," from Latin alibi (adv.) "elsewhere, somewhere else," locative of alius "another, other, different," from PIE root *al- (1) "beyond." The weakened sense of "excuse" is attested since 1912, but technically any proof of innocence that doesn't involve being "elsewhere" is an excuse (n.) and not an alibi.