John Gill Commentary Psalms 51:2

"Wash me thoroughly from mine iniquity, And cleanse me from my sin." — Psalms 51:2 (ASV)

Wash me thoroughly from mine iniquity
Which supposes defilement by sin, and that very great, and such as none can remove but the Lord himself; who, when he takes it in hand, does it effectually and thoroughly; see (Ezekiel 36:25) . David's sin had long lain upon him, the faith of it had as it were eaten into him, and spread itself over him, and therefore he needed much washing: "wash me much", all over, and thoroughly:

and cleanse me from my sin :
which only the blood of Christ can do, (1 John 1:7) . The psalmist makes use of three words to express his sin by, in this verse (Psalms 51:1) ; (evp) , which signifies "rebellion", as all sin has in it rebellion against God the lawgiver, and a contempt of his commandments; (Nwe) , "perverseness", "crookedness", sin being a going out of the plain way of God's righteous law; and (tajx) , "a missing the mark"; going besides it or not coming up to it: and these he makes rise of to set forth the malignity of sin, and the deep sense he had of the exceeding sinfulness of it; and these are the three words used by the Lord in (Exodus 34:7) ; when he declares himself to be a sin forgiving God; so that David's sin came within the reach of pardoning mercy.