Amen comes from a Hebrew root, aleph mem nun, that also gives emunah, faithfulness, and its core sense is something like firm or so be it, an affirmation that what has just been said is trustworthy and stands. What is unusual is not the word's meaning but its total resistance to translation across an extraordinary range of unrelated languages over roughly three thousand years: it passed from Hebrew into Greek in the Septuagint and New Testament, from Greek into Latin in the church's liturgy, from Hebrew and Aramaic separately into Arabic through the Quran's own use of amin in supplication, and from all of these into essentially every vernacular language Judaism, Christianity and Islam have subsequently reached, with the word kept as a loan rather than rendered as so be it or truly, the more literal translations available in most of those languages.
No single decision produced this outcome, and there is no equivalent case in this atlas of a formula surviving unaltered across four such distantly related language families, Hebrew, Greek, Latin and Arabic, plus everything downstream of them, without exception. Linguists studying religious vocabulary generally treat this as evidence that certain ritual response words acquire a kind of performative status where the sound itself, rather than any translation of its content, is what carries the affirming function in worship, so that translating amen into so be it in English liturgy today would arguably not do the same job the untranslated loanword does, even though the literal meaning would be more transparent to a listener unfamiliar with the word's history.