“Purify me with hyssop”
Purify me with hyssop, and I will be clean; wash me, and I will be whiter than snow. (Psalm 51:7 BSB)
What it sounds like today
An odd request to be scrubbed with a random herb.
The cultural key
Hyssop is not a soap or a fragrance but a ritual instrument. The little bushy plant, its clustered leaves able to hold liquid, was bound into a brush and used to sprinkle blood or purifying water in Israel's cleansing rites: it daubed the lamb's blood on the doorposts at Passover (Exodus 12:22), sprinkled the blood that cleansed a healed leper (Leviticus 14), and applied the water of purification to anyone defiled by contact with death (Numbers 19). So when David prays 'purify me with hyssop,' he is reaching for those ceremonies and asking God to do to his guilty soul what the priest's hyssop did to the ritually unclean: declare and make him clean. He wants not a wash but a purification rite performed on his conscience.
Recovered meaning
Hyssop was the ritual brush used to sprinkle cleansing blood or water on the unclean. David asks God to perform on his sinful heart the purification ceremony that hyssop enacted, to make him ceremonially, and truly, clean.