Psalms 51:2

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, Psalms 51:9; and

(b) by making the heart pure and holy through the renewing and sanctifying influences of the Holy Spirit, Psalms 51:10-11.