alibi (n.)

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.