A bunch of hyssop. The hyssop was regarded as having purging or purifying qualities, and was used in the cleansing of the leper (Le Exodus 14:4), and of the leprous house (ibid. 51-52), and also formed an element in the "water of separation" (Numbers 19:6).
It was a species of plant which grew on walls, and was generally low and insignificant (1 Kings 4:33), yet which could furnish a stick or stalk of some length (John 19:29). It must also have been a common plant in Egypt, the wilderness, and Palestine.
Two suggestions are made with respect to it. One, that it was a species of marjoram (Origanum Aegyptiacum, or O. Syriacum ) common in both Egypt and Syria; the other that it was the caper plant (Capparis spinosa), which abounds especially in the Desert.
It is in favour of this latter identification, that the modern Arabic name for the caper plant is asaf or asuf, which excellently represents the Hebrew ezob, the word uniformly rendered in our version by "hyssop" The blood that is in the basin.
The Septuagint and Vulgate render—"that is on the threshold." Saph—the word translated "basin" has the double meaning. None of you shall go out. Moses may well have given this advice on his own authority, without any Divine command.
(See introductory paragraph.) He would feel that beyond the protection of the blood of the lamb, there was no assurance of safety.