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.
else (adv.)youdaoicibaDictYouDict
Old English elles "in another manner, other, otherwise, besides, different," from Proto-Germanic *aljaz (cognates: Gothic aljis "other," Old High German eli-lenti, Old English el-lende, both meaning "in a foreign land;" see also Alsace), an adverbial genitive of the neuter of PIE root *al- (1) "beyond" (cognates: Greek allos "other," Latin alius; see alias (adv.)). As a quasi-adjective, synonymous with other, from 1660s; the nuances of usage are often arbitrary.

Productive of a number of handy compounds that somehow never got traction or have been suffered to fall from use: elsehow (1660s) "somehow or other;" elsewards (adv.), 1882, "somewhere else;" Old English elsewhat (pron.) " something else, anything else;" elsewhen (adv.), early 15c., "at another time; elsewhence (c. 1600); elsewho (1540s). Among the survivors are elsewhere, elsewise. Menacing or else, with omitted but implied threat, is from 1833.