This word did not exist in the Arabic language until it was - as usual - plagiarised by Muhammad from the Hebrew language during his stay in Madina among its 'Jews'.

       It does not appear anywhere in the Quran but is mentioned and described in Ahadith.

       In Hebrew it means So Be It. It occurs 14 times in the Torah as a formula of confirmation or agreement as commanded by Moses.

Deuteronomy 27:15 " Cursed be the man who makes any engraved or molten image, an abomination to the Lord, the work of the hands of the craftsman, and sets it up in secret. And all the people shall answer and say, Amen"

              During the period of the Second Temple, Amen served as a response to benedictions and prayers recited outside the Temple.

        By pronouncing Amen, the listener associates himself with what has been uttered; he makes it his own and is ready to conform to it. It became customary, later on, to conclude every good wish with Amen.

Islamic Terms Dictionary
Amin means custodian or guardian.  Someone who is loyal or faithful.

Sahih Al-Bukhari HadithHadith 1.749        Narrated byAbu Huraira
Allah's Apostle said, "Say 'Amin when the Imam says 'Ghair-il-maghdubi 'alaihim wala-ddal-lin;' not the path of those who earn Your Anger (such as Jews) nor of those who go astray (such as Christians); all the past sins of the person whose saying (of Amin) coincides with that of the angels, will be forgiven.

Sahih Al-Bukhari HadithHadith 5.665        Narrated byAnas
The Prophet said, "Every nation has an Amin (i.e. the most honest man), and the Amin of this nation is Abu 'Ubaida bin Al-Jarrah."

       *** The Hebrew Amen (a prayer formula) is not the same as the Arabic Amin (faithful/loyal/guardian) although Muhammad with his usual ignorance believed that they are ***

Sahih Al-Bukhari HadithHadith 8.411        Narrated byAbu Huraira
The Prophet said, "When the Imam says 'Amin', then you should all say 'Amin', for the angels say 'Amin' at that time, and he whose 'Amin' coincides with the 'Amin'of the angels, all his past sins will be forgiven."

Sahih Muslim HadithHadith 828        Narrated byAbuHurayrah
The Messenger of Allah (peace be upon him) while teaching us (the principles of faith), said: Do not try to go ahead of the Imam, recite takbir when he recites it, and when he says: " Nor of those who err," you should say Amin, bow down when he bows down, and when he says: "Allah listens to him who praises Him," say: "O Allah, our Lord, to Thee be the praise."

Sunan of Abu-DawoodHadith 932        Narrated byWa'il ibn Hujr
When the Apostle of Allah (peace be upon him) recited the verse "Nor of those who go astray" (Surah al-Fatihah, verse 7), he would say Amin; and raised his voice (while uttering this word).

Sunan of Abu-DawoodHadith 938        Narrated byAbuZuhayr an-Numayri
AbuMisbah al-Muqra'i said: We used to sit in the company of AbuZuhayr an-Numayri. He was a companion of the Prophet (peace be upon him), and he used to narrate good traditions. Once a man from among us made a supplication.  He said: End it with the utterance of Amin, for
'Amin' is like a seal on the book.  AbuZuhayr said: I shall tell you about that. We went out with the Apostle of Allah (peace be upon him) one night and came upon a man who made supplication with persistence. The Prophet (peace be upon him) waited to hear him.  The Prophet (peace be upon him) said: He will have done something which guarantees (Paradise for him) if he puts a seal to it.  One of the people asked: What should he use as a seal?  He replied: Amin, for if he ends it with Amin, he will do something which guarantees (Paradise for him).  Then the man who questioned the Prophet (peace be upon him) came to the man who was supplicating, and said to him: So-and-so, end it with Amin and receive the good news. These are the words of Mahmud.

       *** All the good and important concepts, precepts, thoughts and ideas in the Quran, were Plagiarised, Plundered, Pirated and or Perverted from the Bible and Hebrew Scriptures ***