Albert Barnes Commentary Psalms 51:2

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

Wash me thoroughly from my iniquity — literally, “Multiply to wash me.” The word rendered “thoroughly” 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 to remove a stain, with the adverbial sense denoting 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:

1. by forgiveness of the past (Psalms 51:9); and
2. by making the heart pure and holy through the renewing and sanctifying influences of the Holy Spirit (Psalms 51:10–11).