Harvey Guthrie’s Theology as thanksgiving: From Israel’s Psalms to the church’s Eucharist has multiple problems, but I think he gets the meaning of zedek (“righteous”) just right (p. 9):

“the original meaning of zedek may have been been connected with the action of a protector god toward a devotee who called upon that god. Zedek , or zedekah , was what took place when the desperate cry of a human being was heard and responded to by the god to whom the cry had been directed. Zedek did, therefore, denote a ‘saving act,’ a ‘triumph,’ or a ‘victory’ for the god, a ‘vindication’ of both the devotee whose faith had been justified and the god whose reputation as a deliverer had been sustained. Zadik denoted the one who had been saved, the one in whom the god’s victory or triumph had been manifest, the one who had been vindicated by zedek . . . . .What qualifies a person to be designated zadik is not what that person has accomplished, but the gracious initiative of God.”