to be clear of guilt when the question is put, or to keep a heart or a life clear of it
This verb has one kind of dirt in view, and it is not the kind water removes. Every use is about guilt: whether a person still has something standing against him, whether a heart has been kept free of it, whether a judge comes out of a case with nothing to answer for. That is why the word turns so easily in two directions. Sometimes the person is the one who has to come out clean, and sometimes the person is the one doing the cleaning, of his own heart or his own way of living.
Job's friends use it to close a door. Twice they ask what a human being would have to be in order to come out clean in front of God, and both times they set the question beside the word for being in the right (צָדַק), so the two questions collapse into one: can anyone stand up under God's examination? Then, one verse later in each speech, they answer by pointing at the sky, using a look-alike verb (זָכַךְ) to say that not even the stars are clear in God's sight.
Psalm 51 makes the surprising move. There it is God who comes out clean, and the psalmist says so in the middle of confessing his own sin: whatever verdict God hands down, God will be found to be in the right. The same word that asks whether a guilty man can be cleared is used to say that the judge cannot be faulted.
The rest of its uses turn the question into a demand, and each one names what the cleaning actually involves.
- Isaiah tells a whole city to make itself clean, then says plainly what that means: stop doing evil.
- Psalm 119 asks how a young man keeps his path clean, and answers by pointing to God's word.
- Proverbs asks who could claim to have made his own heart clean, and expects nobody to raise a hand.