هُوَ ٱللَّهُ ٱلَّذِى لَآ إِلَـٰهَ إِلَّا هُوَ ٱلۡمَلِكُ ٱلۡقُدُّوسُ ٱلسَّلَـٰمُ ٱلۡمُؤۡمِنُ ٱلۡمُهَيۡمِنُ ٱلۡعَزِيزُ ٱلۡجَبَّارُ ٱلۡمُتَڪَبِّرُۚ سُبۡحَـٰنَ ٱللَّهِ عَمَّا يُشۡرِڪُونَ (٢٣)
He is Allâh beside Whom is Lâ ilâha illa Huwa (none has the right to be worshipped but He) the King, the Holy, the One Free from all defects, the Giver of security, the Watcher over His creatures, the All-Mighty, the Compeller, the Supreme. Glory be to Allâh! (High is He) above all that they associate as partners with Him. (Al Hashr 59: 23)
Al Muhaymeen means
the One who Sustains and protects all that exists. He knows what they do; nothing of their affairs can be hidden from Him:
Neither you (O Muhammad SAW) do any deed nor recite any portion of the Qur’ân, – nor you (mankind) do any deed (good or evil), but We are Witness thereof, when you are doing it. And nothing is hidden from your Lord (so much as) the weight of an atom (or small ant) on the earth or in the heaven. Not what is less than that or what is greater than that but is (written) in a Clear Record. (Tafsir At-Tabarî). (Yunus 10: 61)
also it means the Witness
Ibn Abbas and other scholars said, “Muhaymeen means the Witness over His creatures for what they did. He is the Observer and Watcher of what they do”
On the Day when Allâh will resurrect them all together (i.e. on the Day of Resurrection) and inform them of what they did. Allâh has kept account of it, while they have forgotten it. And Allâh is Witness over all things. (Al Mujadila 58: 6)
Whether We show you (in your lifetime, O Muhammad SAW) some of what We promise them (the torment), – or We cause you to die, – still unto Us is their return, and moreover Allâh is Witness over what they used to do. (Yunus 10: 46)
The word for witness in Arabic is شَہِيدٌ
Al Khattabi explained the meaning of Al Muhaymeen as
The One Who bears witness over all that His creatures do or say. It was also said that Muhaymeen means the Observer and Protector of every thing.
Allah also describes His Book the Quran, as being Muhaymeen
وَأَنزَلۡنَآ إِلَيۡكَ ٱلۡكِتَـٰبَ بِٱلۡحَقِّ مُصَدِّقً۬ا لِّمَا بَيۡنَ يَدَيۡهِ مِنَ ٱلۡڪِتَـٰبِ وَمُهَيۡمِنًا عَلَيۡهِۖ
And We have sent down to you (O Muhammad SAW) the Book (this Qur’ân) in truth, confirming the Scripture that came before it and Muhaymin (trustworthy in highness and a witness) over it (old Scriptures) Al maidah 5: 48
The Quran is a witness over the previously revealed books.
It contained the best of each book and abolished whatever did not adhere to its rules and provisions. It also revealed the interpolations made by His enemies, teh jews. Furthermore, the Quran indicated the truth embodied in the previous books.