Before ancient sacrificial rites, one had to bathe and fast to cleanse oneself of foul qi.
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usage
秽 is a classical/literary bound form — it almost never appears alone in modern Chinese. It compounds into words like 污秽 (filth), 秽语 (obscene language), 淫秽 (obscene/indecent), and 自惭形秽 (to feel ashamed of oneself).
