AL-QUDDUS-The Absolutely Pure The Most Sacred

The One who is Very Pure, Absolutely Pure. He is far and above any imperfection, error, fault and shortcoming. He is High above what does not befit Him. He is the one who is Taahir (Pure) and Uluww (High). The One who is pure from any imperfection and clear from children and adversaries.
Allah, Exalted be He, says, “All that is in the heavens and all that is in the earth glorify Allah, the Sovereign Lord, the Holy One, the Exalted in Mighty, the All-Wise..” (Surat al-Jumua’h, 62:1)
Al-Quddoos means the One who is free of any imperfection, and evil is not attributed to Him, for all perfect and sublime attributes belong to Him.

Benefits of belief in this name include the following:

Loving, Allah, Exalted be He, extolling Him and venerating Him becauseHe is free of all imperfections.
Declaring Allah free of all imperfections with regard to His Supreme Being, statements, actions, names and attributes and considering this an act of worship. This involves the following:
Affirming all the names and attributes of Allah, Exalted be He, which He has confirmed for Himself or confirmed by His Messenger (may Allah bless him and grant him peace) without changing their meanings, denying them completely or likening them to those of any of His creatures.
Declaring Allah, Exalted be He, free of having any partners, an equal, a wife or children.
Referring all judgements to His law, implementing it, submitting completely to it, wholeheartedly accepting it, for it is free of imperfection, injustice and ignorance.
Not entertaining any evil thoughts about the Lord of all the worlds. Allah, Exalted be He, says, censuring the unbelievers and the hypocrites, “…those who think evil thoughts of Allah; against them shall be the evil turn of fortune.” (Surat A-Fath, 48:6) Therefore, any thought about Allah Almighty which does not befit His majesty in reality contradicts His name Al-Quddoos.

The One who is pure, spotless, without blemish or fault.

The One who is far from, and untouched by, worldly imperfections or faults.

The One whose essence and attributes are of unimaginable purity and perfection.

The One who is beyond all human understanding of purity, perfection and holiness.

From the root q-d-s which has the following classical Arabic connotations:

to be pure, clean, spotless
to be far removed from impurity or imperfection
to be holy, sacred, hallowed

This name is used in the Qur’ān. For example, see 59:23

The ancient roots of the Arabic word Quddūs, as well as the Hebrew word Kadosh, point toward that which is set apart, different, that which is ‘other’. Thus, al-Quddūs is known as the One who is set apart, distinctly different and separate from all worldly imperfections, sins and faults, the One whose perfection and righteousness are so ‘other’ that they cannot be grasped by mankind, the truly Blessed One.

The name al-Quddūs describes the unique, unimaginable purity and perfection that is Allāh, the One whose goodness and righteousness are beyond our understanding.

According to al-Qurturbi, the name al-Quddūs also describes the One who is glorified and revered by the angels.

The Merriam-Webster dictionary defines the word holy as:

Holy — Exalted or worthy of complete devotion as one perfect in goodness and righteousness.