Latin 'alibī,' the locative/adverbial form of 'alius' (other), meaning 'elsewhere' — the legal claim of having been in another place.
A claim or piece of evidence that one was elsewhere when an act is alleged to have taken place; more broadly, any excuse or pretext.
Directly from Latin 'alibī,' an adverb meaning 'elsewhere,' the locative form of 'alius' (other, another), from PIE *h₂el- (other, beyond). In Roman law 'alibī' was used as an adverb in pleadings: the accused claimed to have been 'alibī' — elsewhere — at the time of the offence. English adopted the Latin adverb wholesale as a noun, one
The word 'alibi' is one of a very small number of English nouns that were once Latin adverbs. Latin had no word 'alibi' as a noun — it simply meant 'elsewhere.' English lawyers adopted the entire adverb and turned it into a noun, a grammatical transformation that would have puzzled a Roman jurist.