For He has clothed me with garments of salvation and arrayed me in a robe of righteousness, as a bridegroom adorns his head like a priest, and as a bride adorns herself with her jewels. Whatever the fig leaves, whether made of self-esteem therapy or of energetic moralism, we refuse to see our filthy rags as insufficient to appear in the presence of a holy God. God's reckoning of righteousness to us is not a matter of him searching out our hearts and finding righteousness within us. That is not imputation. He would find only filthy rags.
Imputation is a reckoning or a putting to the account of.
The idea is one of an accounting of righteousness from one who is truly righteous to one who is bankrupt of all righteousness. So if God is imputing righteousness to us, it is not our own but another's. God does not whip up righteousness out of thin air. It is Christ's real righteousness that is imputed to us by faith. Then this is your good news, that God in Christ has done for you what you cannot do yourself.