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.
“There is nothing like Him.”
[ash-Shura (42): 11]
“There is no one equal to or comparable to Him.”
[al-Ikhlas (112): 4]
“Do you know of any who is similar to Him?”
[Maryam (19): 65]
“Whatsoever is in the heavens and the earth glorifies Allah - the King, al-Quddus, the Almighty, the All-Wise.”
[al-Jumu'ah (62): 1]
From “Explanation to the Beautiful and Perfect Names of Allah” — Shaykh as-Sa’di (d.1376H)