hyssop

Boyd’s Bible Dictionary:

(aromatic plant). A bushy herb, of the mint family (Ex. 12:22; Lev. 14:4,6,51; 1 Kings 4:33; John 19:29).

Concise Bible Dictionary:

Hyssop
A plant used in the Jewish observances. It is often associated with cedar wood and scarlet, and was used in sprinkling the unclean. It is described as springing out of the wall, showing its littleness, though some of its stems may have been long enough to be used as a reed on which the sponge was placed to give the Lord vinegar when on the cross. Others however suppose that the hyssop was added because of its aroma, and both the sponge and the hyssop were tied to a reed. The hyssop is in contrast to the stately cedar, and is symbolical of man’s littleness. The words ezob and ὔσσωπος doubtless point to the well-known hyssop which is extensive in Palestine, though some suppose other aromatic plants are included, as the wild marjoram (Ex. 12:22; Lev. 14:4,6,51-52; Num. 19:6,18; 1 Kings 4:33; Psa. 51:7; John 19:29; Heb. 9:19).
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Strong’s Dictionary of Hebrew Words:

Transliteration:
’ezowb
Phonic:
ay-zobe’
Meaning:
probably of foreign derivation; hyssop
KJV Usage:
hyssop

From Manners and Customs of the Bible:

Psalm 51:7. Purge me with hyssop, and I shall be clean.
Hyssop was appointed to be used in ceremonial purification. It was used in connection with the passover (Ex. 12:22), the cleansing of lepers (Lev. 14:4,6,49,51,52), and the sacrifice of the red heifer (Num. 19:6,18). See also Hebrews 9:19. Hyssop was anciently considered a means of actual bodily purification, and was even taken internally for that purpose.