In ancient times people used milfoil stalks to divine good or bad fortune.
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筮 is not used independently in modern Chinese. It survives in classical pairings like 卜筮 (divination by shell-cracking and by stalks) and 筮仕 (to take up one's first official post, after the divination once cast for it).
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Archaic and literary only, seen in old ritual texts and etymology notes, not in everyday speech.