I forgot, or more likely never knew, the meaning of אזוב (Exodus 12:22, שמות פרשת בא, ולקחתם אגדת אזוב). Onkelos translates it unhelpfully as איזובא. Saadiah Gaon says it means זעתר (za'athar), which Ibn Ezra translates as אוריגנו (origano/oreygano). (Ibn Ezra says of Saadiah's gloss that it "isn't likely" [זה לא יתכן] for Biblical and botanical reasons, but admits that he doesn't know what plant the word refers to.) Yidish-taytsh, I think, calls it אזובֿ, as does Yehoash.
The Septuagint renders the word as hyssopos 'hyssop.' Here's where I quote William H.C. Propp in Anchor Bible Exodus 1-18, where he chooses to call it "marjoram":
The LXX rendering . . . is based on the phonetic similarity to Hebrew 'ezob, a common translation technique for LXX (Tov 1979). But even when Hebrew and Greek words are truly related via borrowing, their meanings may differ slightly. 'Ezob is "Syrian hyssop" (Origanum syriacum l.) or marjoram, still used by Samaritans for Pesakh (Saadiah; Crowfoot and Baldensperger 1931). Greek hyssopos is a different plant altogether (Zohary 1982:96). A literalist might object that we should seek a native Egyptian plant, rather than a Palestinian, but no doubt later Israelite practice has been transplanted into an Egyptian setting.
The marjoram tuft, readily available (cf. 1 Kgs 5:13), is essentially a brush (Cassuto 1967:143 [this is also implied in the commentaries of Ibn Ezra and Saadiah -- ZB]). Segal (1963:159) and Jacob (1992:328) plausibly infer that it insulates lay officiants from the dangers of the holy; true priests, in contrast, manipulate blood with their fingers. Marjoram is also used to apply blood in ritual purifications (Ps 51:9), particularly from skin disease (Leviticus 14 passim) and death (Num 19:18, cf v 6). According to Heb 9:19, it was also used for the Covenant ceremony of Exod 24:8.
What relationship Saadiah's term za'athar has to modern-day zaatar, and why marjoram is called Origanum (so what's oregano, then?), are puzzles for wiser minds to solve.
Oh, here's a useful note:
In the countries of the Eastern Mediterranean, there is often no clear distinction made between a couple of aromatic herbs of the mint family: Names like Turkish kekik or Arabic zatar/satar [زعتر, صعتر] and related forms in Hebrew and Persian, often in conjunction with qualifying or descriptive adjectives, may be applied to a varity of native herbs including, but not restricted to, oregano, marjoram, thyme and savory. Usage may vary even within a given language, depending on the region and particularly on the local flora. In Jordan, zahtar usually means a spice mixture containing such herbs (see sumac for more).