noun (plural alibis)
verb (alibis, alibied, alibiing)[with object] informal
late 17th century (as an adverb in the sense 'elsewhere'): from Latin, 'in another place; elsewhere'. The noun use dates from the late 18th century
The weakened nonlegal use of alibi to mean simply ‘an excuse’ is a fairly common and natural extension of the core meaning. It is acceptable in standard English, although regarded as incorrect by some traditionalists.