The Hebrew verb zakak means to be pure, bright, or transparent — like clear water or polished metal. It is used both of physical purity and moral uprightness. The root conveys the idea of clarity with nothing hidden or contaminating.
Zakak appears in Job's famous declaration of integrity (Job 15:15; 25:5) and in Lamentations' lament over the dulling of Israel's purity. It speaks to a quality that only God possesses absolutely — the heavens themselves are not pure in His sight (Job 15:15). This humbles human pride: if creation's brightest lights cannot be pure before God, how much more must humans depend on His grace for righteousness. The image points forward to Christ, through whom we are cleansed and made pure (1 John 1:7).