Apparently no one’s really sure, biblical scholars argue about it, and it might be the same thing as hyssop.
Ezov (Hebrew: אזוב) is the Classical Hebrew name of a plant mentioned in the Bible in the context of religious rituals. In some English-language Bibles, the word is transliterated as ezob.
TL;DR: It might be hyssop, it might be “various local herbs, including marjoram, oregano and thyme”, and it might be capers. Mmm. Capers.
I don’t think capers have medicinal properties, but at this point I think McCaffrey was probably as tired of this sentence as I am right now.