Wash me throughly from mine iniquity - literally, "Multiply to wash me." The word rendered "throughly" is a verb, either in the infinitive or imperative mood, and suggests the idea of "multiplying" or "increasing." The reference is to that which might need constant or repeated washings in order to remove a stain adverbially to denote intensity, or thoroughness. On the word wash as applicable to sin, see the notes at Isaiah 1:16.
And cleanse me from my sin - Remove it entirely. Make me wholly pure. See the notes at Isaiah 1:16. In what manner he hoped that this would be done is shown in the following portions of the psalm. It was -
(a) by forgiveness of the past, Psalm 51:9; and
(b) by making the heart pure and holy through the renewing and sanctifying influences of the Holy Spirit, Psalm 51:10-11.

Wash me throughly - הרבה כבסני harbeh cabbeseni, "Wash me again and again, - cause my washings to be multiplied." My stain is deep; ordinary purgation will not be sufficient.

Wash me (c) throughly from mine iniquity, and cleanse me from my sin.
(c) My sins strike so fast in me, that I have need of some singular kind of washing.

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 Psalm 51:1; which signifies "rebellion", as all sin has in it rebellion against God the lawgiver, and a contempt of his commandments; "perverseness", "crookedness", sin being a going out of the plain way of God's righteous law; and "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.

Wash me--Purity as well as pardon is desired by true penitents.