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 Eze 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, 1Jo 1:7.