alibiyoudaoicibaDictYouDict[alibi 词源字典]
alibi: [18] In Latin, alibi means literally ‘somewhere else’. It is the locative form (that is, the form expressing place) of the pronoun alius ‘other’ (which is related to Greek allos ‘other’ and English else). When first introduced into English it was used in legal contexts as an adverb, meaning, as in Latin, ‘elsewhere’: ‘The prisoner had little to say in his defence; he endeavoured to prove himself Alibi’, John Arbuthnot, Law is a bottomless pit 1727.

But by the end of the 18th century it had become a noun, ‘plea of being elsewhere at the time of a crime’. The more general sense of an ‘excuse’ developed in the 20th century. Another legal offspring of Latin alius is alias. This was a direct 16th-century borrowing of Latin aliās, a form of alius meaning ‘otherwise’.
=> alias, else[alibi etymology, alibi origin, 英语词源]
alibi (n.)youdaoicibaDictYouDict
1743, "the plea of having been elsewhere when an action took place," from Latin alibi "elsewhere, somewhere else," locative of alius "(an)other" (see alias (adv.)). The weakened sense of "excuse" is attested since 1912, but technically any proof of innocence that doesn't involve being "elsewhere" is an excuse, not an alibi.