The one who is far greater and far removed from any imperfection, or that He resemble any of His creation. Hence He is far removed from any defect just as He is far removed from anything resembling Him or coming close to resembling Him in any of His Attributes of perfection. Al-Quddus is similar in meaning to as-Salam in that they both negate any form of imperfection while at the same time including unlimited perfection in every way. This is because when deficiency has been removed then all that remains is perfection.
“He is Allah, other than whom none has the right to be worshipped. The King, the Holy, as-Salam, the Giver of Security, the Ever-Watcher, the Almighty, the Compeller, the Supreme. Glory be to Allah! [High is He] above all that they associate as partners with Him.”
[al-Hashr (59): 23]
From “Explanation to the Beautiful and Perfect Names of Allah” — Shaykh as-Sa’di (d.1376H)