The first occurrence.
Of every clean beast you shall take to yourself by sevens, the male and his female: and of beasts that are not clean by two, the male and his female. Gen 7:2
The primitive root.
Strong’s H2889 tahowr, an adjective meaning, “clean, pure;” from Strong’s H2891 טהר taher, a primitive root meaning, “to shine, to be bright.” The 3-letter root is tet + hey + resh.
tet ט = the basket, thus surround, contain, hold, mud
hey ה = the man with upraised arms, thus look, reveal, wonder, worship, breath
resh ר = the head of man, thus head, first, top, beginning, man
Tet can sometimes signify mud, as baskets were most often used to carry mud. The story: When mud (tet) or tarnish covers an item, polishing it so that it shines (taher) reveals it (hey) as it was at first (resh).
One cognate word from this verbal root is prison, the place that contains or holds (tet) the man (resh). The purpose of the prison is to either purify society from lawbreakers, by removing them, or purify the man from lawbreaking, by removing his freedom and hopefully cause him to reflect on why he is there; so that he leaves off folly and gains wisdom.