Abarim Publications' online Biblical Hebrew Dictionary
The verb אזב ('zb) isn't used in the Bible but here at Abarim Publications we suspect that it has some relationship with the verb זבב (zbb), meaning to zip to and fro (what flies do). But from our verb אזב ('zb) comes the masculine noun אזוב ('ezob), which is a word for a plant with proverbial tiny stems, leaves and flowers, that typically flutter in the wind (or sprout up everywhere, even on walls; 1 Kings 4:33).
The English translation of this noun is really not a translation but rather a transliteration: hyssop — the same happened to מור, mor, which became our word "myrrh" — and even though the name hyssop became applied to a particular plant (namely Hyssopus officinalis) no certainty exists as to the genus of the Biblical 'ezob. A very good reason for this is that in the ancient world creatures weren't named after their scientific genus but after their defining behavior or purpose, which in this case would have been described by the allusive verb אזב ('zb) and was doubtlessly shared among several species of plants.
And to take this argument even further: the defining quality of Biblical 'ezob was that it purified — as David famously wrote: "Purify me with hyssop, and I shall be clean; wash me, and I shall be whiter than snow" (Psalm 51:7). In other words: 'ezob was the old world's soap (and not a plant genus).
Our English word "soap" probably comes from an old Germanic stem, with a root so old that it also existed in Latin (as sebum) and lives on today in most European languages (Basque: xaboi; Bosnian: sabo; Danish: saebe; Dutch: zeep; German: Seife; Estonian: seep; Spanish: jabon; French: savon; Finnish: saippua; Icelandic: sapa; Kurdish: sabun). Its prevalence across such a wide language base make linguists suspect that it existed in pre-Indo-European (probably sounding like seib), and that would make it not only contemporary with our word 'ezob; it may very well be its cognate, and thus that of the Greek noun υσσωπος (hussopos), which appears in the New Testament as the vehicle with which the dying Jesus was fed wine (John 9:29).
The use of soap, then and now, had to do with vigorous scrubbing, which might explain the suggested relation with the verb זבב (zbb), meaning to move to and fro.
Just before the Exodus, the Israelites were told to slaughter a lamb and apply its blood to the door posts and lintel by means of a sponge-like item (אגדה, agudda) made from hyssop (Exodus 12:22). The previous nine plagues that hit Egypt are clearly associable with infections, diseases and dirtiness, and to avert the deadly culmination of this series, the use of soap would be more than simply a sign.
A lavish use of אזוב ('ezob) is prescribed in dealing with צרעת, (sara'at), which literally means "being spotted" or "having an unevenly colored skin" (Leviticus 14). A mottled skin could be the symptom of a horrible disease, but it could also mean that the person was due for a good wash. Being sick would mean exile and possibly a slow and lonely death, so the fear of being found with it had the Israelites scrubbing at every turn, and that kept the majority healthy.
Numbers 19 describes a major element in Israel's hygiene laws: a red heifer (פרה אדמה, para adamah) was to be burned to ash, after which several items were added, including hyssop (Numbers 19:6). The resulting concoction was gathered and stored, "and the congregation of the sons of Israel shall keep it as water to remove impurity..". (19:9). This "water for impurity" was then spritzed on everything that might be dirty, from people to pots and pans (19:15), in some specific cases with additional flowing water (19:17).
Besides the various theological implications of these procedures, the Israelites were obviously simply making soap.
Soap in the Old World
We moderns are so used to soap that we forget that to the ancients it must have seemed like a miracle elixir, so potent and so effective that even the deity heeded it. Soap was a shield that could stave off plagues; a divine potion that meant the difference between life and death. We moderns are so used to religion that we forget that to the ancients everything was religion, and that all ritual and theology was first and foremost rooted in reality and survival. We wash our hands without thinking twice, but to the ancients, soap was what absorbed iniquities and flushed them away in a cleansing baptism.
Liquid soaps were made in Babylon as early as the third millennium BC, by mixing animal fat with ashes. Its miraculous properties were universally recognized and Babylonian folks vigorously scrubbed up before and after each meal, as "uncleanliness was considered a cause of disease" (says Robert Paulissian, M.D. in his Medicine in Ancient Assyria and Babylonia).
All soaps essentially are based on a molecule with one end made from fat and the other from salt. When soap mixes with grease, the fatty end binds to the grease and the salty end dissolves in water. A few buckets of the latter easily flushes the grease-soap residue away, much to the joy of those who like things clean.
Our word אזוב ('ezob) probably denoted the vehicle of the cleaning agent; a sponge- or scrubber-like thing of interwoven branches, that carried the cleaning agent. This cleaning agent, the actual soap, appears to have been known as the "blood" (דם, dam), which explains the lavish dispensing of blood where the objective obviously was to cleanse.
How mammalian blood can be used to clean is a mystery, but there is very little room to maneuver about the wording: the blood appears to have been actual blood and not in some way "symbolical" blood. This blood came dripping out of a lamb or a bird, and they were real enough to be subsequently eaten. But apparently, people did actually use blood for cleaning, and this blood didn't turn things red: "they have washed their robes and made them white in the blood of the Lamb" (Revelation 7:14).
This suggests that the word for blood (דם, dam) was not restricted to what we call blood today, but incorporated other liquids as well. It may even be that soap was discovered (in stead of invented out of the blue) in an animal that suffered from pancreatitis. As Thomas H. McConnel writes in The Nature of Disease: "In pancreatitis, soap is made from normal fat in and around the pancreas and from calcium salts present in biliary and pancreatic juices and blood. Pancreatic juice is alkaline and combines the calcium salts and fat to make soap".
This natural soap production is called fat necrosis and occurs, apart from in pancreatitis, as a result of trauma or infection and commonly in the breast and subcutaneous areas (says Mosby's Medical Dictionary). All this suggests a whole new perspective to the already mind-boggling words of Isaiah 53:4.