Psalms 51:2 - Coke's Commentary on the Holy Bible

Wash me thoroughly, &c.— The original כבסני ברבה hereb kabseini is, multiply, or, in multiplying, wash me from mine iniquity, and cleanse me from my sin; for the word multiply refers to both verbs, wash me and cleanse me, and is well rendered in our version by thoroughly wash me; as a garment often washed is thoroughly cleansed from its impurity. This form of expression is frequent in the Old Testament. See Isaiah 1:16. The meaning of the Psalmist is, that God, by repentance and faith, would recover him from all his past transgressions, and enable him to live free from the practice of them for the future.